Jargon is the set of terms and expressions that have defined meaning within a field but lack that meaning (or perhaps any meaning) outside that field. Computing’s jargon is primarily based on English words with computing-specific meanings which may or may not be related to the English meanings of those words.
On this page are terms I have defined in class and which I expect you to understand.
Some kinds of information can be stored in multiple ways, with sometimes nuanced difference in how the different data structures that store it perform. An abstract data type provides an abstraction barrier, allowing algorithms to be designed ignoring the details of the data structure chosen.
Practically, an abstract data type consist of a set of named algorithms (i.e. functions) that perform specific tasks on the data. For example, an abstract data type representing a queue of things waiting their turn might have functions for add to back of line
and remove from front of line
. And data structure for which those functions can be created is said to implement the abstract data type.
Abstraction strong enough that we can use the abstraction without thinking about the details it hides.
No abstraction barrier is perfect: there are always limitations of nuances which do matter in some cases. For example, Ariane flight V88 exploded and a Patriot Missile hit the army that fired it both because the abstraction barrier between bits and numbers failed.
Unambiguously-defined steps to accomplish some task.
Algorithms are generally described in terms of the problem they solve (or approximately solve in the case of heuristics), the data used to encode that problems inputs, and the steps used to compute the answer.
The word algorithm
is a Latinization of the name of Muḥammad ibn Mūsā al-Khwārizmī who wrote books that helped Europeans adopt the ideas of algorithms with place-value numbers. Algorithms themselves long predate his work.
Often used to mean simply not digital
, or the part of the world that is not computers.
Sometimes used to mean a signal with a meaning based on being analogous to (directly or naturally representing, not using symbols but more intrinsic properties like quantity or frequency) the thing is describing.
Concrete values provided to a function or program in place of its parameters.
The square root function has one parameter, the number to find the root of. In , the number is the argument to that function.
A field within Computer Science that works on algorithms that humans find to be intelligent.
Exactly what that means changes over time. Currently, it is seen to consist of
assembly codeor
assembler code.
Identifying what an authenticated user is permitted to do.
Course-grained authorization might look like a firewall: if you’re permitted inside, you can do anything. Fine-grained authorization might look like an access control list: each user, resource, and action grouping might have its own permission control.
The principle of least privilege is a widely-cited guide to picking an authorization scheme.
Place-value number systems are defined by a single integer called their base. The base tells us
The most popular bases are:
Multiple meanings:
When it is necessary to distinguish that a number is written in binary, it is traditional to precede it with 0b; thus 0b1101 means a binary number with digits 1101 and represents the value thirteen.
A single digit in base 2: either 0 or 1.
The word bit
is a portmanteau of binary digit
.
A family of related technologies for implementing a distributed indelible ledger, where in this context those words mean
Each block consists of the hash of the preceding block, a transaction (typically moving virtual currency between accounts), and a signature from each user losing resources in the transaction.
The chain of blocks found by starting from the most recent block and using hashes to identify each preceding block gives the full state of the blockchain.
Most blockchains allow anonymous users and prevent one user from overwhelming consensus with a large number of voting accounts by making votes cost, typically using either proof of work or proof of stake.
A large number of computers connected to the Internet, all compromised and at least partially controlled by a single cybercriminal.
Botnets often target small devices that are often less secure and less often updated, like smart thermostats, smart doorbells, smart refrigerators, and other Internet-connected appliances. Phones and computes that are not regularly updated are also often controlled by botnets.
Botnets are usually used to attack others rather than to harm the owners of the devices they are made of, for example by launching DDoS attacks.
A characteristic of software, describing the difficulty of making changes or the likelihood that small-sounding changes will either break existing systems or require a large amount of work.
Brittleness is usually accumulated over time as a form of technical debt.
A group of 8 bits. Equivalently, a group of 2 hex digits. Equivalently, a number between 0 and 255 (inclusive).
When it is necessary to write out a byte for human viewing, it is traditional to write it as two hex digits; thus 0D means a byte with the value thirteen and F3 means a byte with the value two hundred forty-three.
As a verb, storing values to prevent the need to compute them again in the future. As a noun, the place where those values are stored.
Caching is widely used to speed up operations that might otherwise be repetitive, such as DNS lookups or the bytes that make up image files used on many webpages.
A family of AI tasks where the output is categorical and the set of possible outputs is known during training. This is generally supervised learning.
Examples might be pick the next word
or tell which person is shown in this image.
A family of AI tasks where the output is categorical and the set of possible outputs is not specified but instead learned by the AI system. This is a form of unsupervised learning.
Examples might be organize these images into groups of similar images
– the number of groups and what kinds of images belong in each is learned, not provided in the training data.
In general, any document signed by a trusted party.
Most often, a document that says the public key used by the legitimate owner of a specific URL.
Certificates always include expiration dates because cracking signatures is a matter of time and computing power; adding an expiration date keeps people from using the same certificate long enough for others to crack it.
The name for the output of an encryption (usually a symmetric encryption) and the input of the associated decryption.
Ideally, it should be nearly impossible to convert from ciphertext to plaintext without knowing the encryption key.
A name for applications and data that are hosted on shared computers rented by cloud providers to multiple users.
Cloud hosting allows averaging out bursty usage patterns that most apps experience between many apps, keeping each server more uniformly busy and allowing fewer overall servers than if each app owned their own.
Cloud providers often use virutal machines or containers to issolate different users’ code and data.
A program that converts source code written in a programming language into a sequence of machine instructions that can be executed by a specific type of computer processor.
Typically, programming languages that use a compiler provide fewer abstractions and thus require more effort to program in than those that use an interpreter; but they also tend to create programs that run more quickly than those created for an interpreter.
A property of secure communications that prevents people other than the intended recipient from knowing the contents of the communication.
The most common tools for achieving confidentiality are symmetric ciphers.
A lighter-weight way of isolating programs and providing the illusion of each running on its own computer than a virtual machine.
In a container, each system call is checked by the container program and adjusted to simulate being on its own computer, but is then handled by the kernel of the computer running the container. This means (for example) that a program running in a container might have every effort to open a file pretend that one directory on the computer is the entire file system, preventing the program from even being able to refer to other program’s files at all.
Containers are widely used in cloud-based computing.
A collection of websites that are only accessible via Tor and are not linked to from more visible Internet sites.
Pages on the dark web are called onion sites
and may be hosted by any Internet-connected device, and can be created and changed without notice or centralized control. Their hostnames are 56 random characters followed by .onion; these URLs are not accessible except through Tor.
A defined, unambiguous representation of information.
Ways of representing simple information like numbers are commonly called data types
or primitive data types
. Ways of representing multipart information like lists are commonly called data structures.
A way of storing data that consists of multiple related but distinct pieces of information.
Most data structures are designed to implement some abstract data type. For example, the data structure array
is implemented by putting one value after another in memory and is one implementation of the abstract data type list
. List
could also be implemented with other data structures, such as by having each element in the list store both a value and the location in memory to find the element that comes after it.
Short for distributed denial of service
, a way of bringing down even secure websites through too many requests.
Typically, DDoS attacks involve an attacker using a botnet to send millions of requests from thousands of IP addresses across the world to a single website, overwhelming its servers without them being able to easily pick out which originating IP addresses are to blame.
One of two common forms of abstraction.
The most common form of detail removal is to refer to a complicated set of actions by name instead of by the actions themselves. Functions are the primary tool for that form of detail removal.
Another important form of detail removal is an abstract data type, which helps use build algorithms that do not depend too closely on the specifics of how we represent data.
A way for two parties to agree on a shared secret, such as the key used in a symmetric cipher, with no prior shared information over an insecure communication channel.
The exchange uses three random numbers and one hard-to-invert function. One number is shared, and one is kept secret by each party. Both parties apply the function to the two numbers they know (their secret number and the shared number) and share the results, then apply it again to their secret and what the other person shared. With the correct choice of function, this resilts in both having the same number at the end.
Represented or operating using a set of discrete symbols called digits, rather than using continuous or analog signals.
Because of the prevalence of programmable general-purpose digital computers, digital
is sometimes used as a synonym for done with computers
.
Short for Domain Name System,
a hierarchical mapping between domains and IP addresses. DNS also contains other related information, such as entries stating that one hostname is an alias of another.
DNS lookups proceed with one query per domain in the hostname, starting with the last. Each request asks for an IP address of a subdomain; for example, to get the IP address of grainger.illinois.edu we ask illinois.edu for it.
DNS is frequently cached to avoid needing to send all these IP lookup requests repeatedly.
A valid suffix of a hostname. For example, in the hostname courses.grainger.illinois.edu the domains are edu, illinois.edu, grainger.illinois.edu, and courses.grainger.illinois.edu.
Typically, DNS performs one IP lookup per domain.
Formally: load the machine instructions of a compiled program into memory and have the processor begin following those instructions.
Informally, also used for running an interpreter on a program’s source code or, less often, for invoking a function.
A concrete use of a vulnerability, in most cases to enact crime.
The possibility that an attacker could take control of my machine using a flaw in one of its systems is a vulnerability. The specific tools they use to do so is an exploit.
Short for floating-point number
, a float is the binary version of scientific notation stored as three parts:
significandor
mantissa)
For example, 56⅔, which in binary is 111000.1010101010101010…, stored as a float with 8-bit significand would be +1.1100010 × 2101 (or in decimal as 1.765625 × 25).
A tool in programming to name a set of actions and use them later by name.
A function definition defines the actions and the name used to refer to them. Many function definitions include parameters.
A function invocation requests that the actions in a previously-defined function be performed. If the function definition included parameters, the function invocation includes arguments to use for those parameters this time.
The phrase call a function
or function call
is synonymous with invoke a function
or function invocation,
but with a stronger verbal abstraction barrier, referring to the function as if it were some external entity you call up instead of a set of actions you take.
An abstraction of a group of transistors that implement some simple operation on a few bits.
The most commonly discussed gates are and, or, not, and xor. For some specific hardware cases, nand and nor are also used.
A computer that is part of the Internet, engaging in routing messages between computers using IP; and is also part of a network of computers that are not part of the Internet themselves but gain access to it through the gateway.
Gateways are the component of ISPs that separates them from other networks.
One of the key qualifiers of a computer, indicating that the computer hardware is capable of doing any information-processing task that any other computer can do.
The Church-Turing thesis posits the existence of general-purpose computers, and of related Turing-complete systems and forms of expression that are capable of describing the operation of general-purpose computers. While not formally proven, this thesis is widely accepted as true.
One consequence of the fact that almost all computers are general-purpose is that new computer technologies (like quantum computers) might differ from others in efficiency and speed, but not in the set of tasks they could eventually do, given unlimited time and storage.
One of two common forms of abstraction.
The most common form of generalization is by adding parameters to a function. For example, is a one-parameter function that finds a square root; is a two-parameter function that generalizes to also compute other roots.
Another common form of generalization is to add parameters to a data structure; for example by defining a generic list of
structure instead of a specific list of integers
structure. Parameterized data structures have more variation in how they are implemented between programming languages than do parameterized functions, leading to them having many names: generics, templates, type parameters, mixins, etc.
gigameaning one billion (G) followed by the abbreviation for Hertz (Hz) which means
per second,together meaning
one billion per second.Commonly used to measure the clock speed of processors, roughly meaning how many billion arithmetic problems the processor can complete per second.
A function that turns arbitrary bytes into a fixed-size number where any change to the input has (with high probability) a different resulting number.
Cryptographically secure hashes are quite large and it is very hard to find two inputs that give the same output. This allows these hashes to be used as unique fingerprints of arbitrary data, making them useful for signatures and blockchains as well as a tool for verifying the integrity of data.
An algorithm that (usually) only approximates the solution to a problem rather than finding the full solution.
Heuristics are more often applied to problems that seek the best of many options, and return not the best option possible, but a pretty good option.
Most heuristics return the optimal output for some inputs but not for most and may return quite poor outputs for a few particularly bad inputs.
A group of 4 bits. Equivalently, a number between 0 and 15 (inclusive).
Hexadecimal digits are typically represented as 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 (with the same meanings they have in decimal) and A, B, C, D, E, F (meaning ten through fifteen in the order listed). Capitalization of the letters is not important:
When it is necessary to distinguish that a number is written in hexadecimal, it is traditional to precede it with 0x; thus 0x10E means a hexadecimal number with digits 10E and represents the value two hundred seventy.
A technique used to train a machine learning model by
On part of a URL, in particular being between the :// and the following /. The hostname identifies a single computer and can be converted to an IP address using DNS.
Because every hostname is a valid domain, it is common for domain
to be used as a synonym for hostname
when the difference is not important.
Usually, information
is used in a loose, human way to refer to the human-understood meaning carried by data.
The information that I am 20 years old could be conveyed by various data: the text I’m 20
, the bits 00010100 in a location where age values are expected, the difference between the current date and a birthdate, etc.
Less often, information
is used as a technical term for the bits of some digital communication that could not be anticipated, an important concept in designing efficient networking, compression, and storage technologies.
A property of secure communications that ensures messages are not modified in transit, or at a minimum that if they are that change can be detected.
The most common tools for achieving confidentiality are hashes (which detect changes) and symmetric ciphers (with make making a specific change almost impossible).
The Internet is a collection of computers using IP and DNS to connect computers in a mostly-decentralized way.
It is common to capitalized Internet
when it is referring to the Internet and leave internet
lower-case when it is used as an adjective. It is also possible (though uncommon) to use internet
(lower-case) to refer to connections between different LANs even when those connections are not part of the Internet (upper-case).
A program that reads the source code of a programming language and takes the actions described by each line of that code.
Typically, programming languages that use an interpreter run much more slowly than those that use a compiler; but they also tend to provide more abstractions and thus require less effort to program in than those created for a compiler.
Short for internet protocol
, a way for computers that are not owned by the same agency to send messages between each other.
There are two current versions of IP: IPv4 assigns each computer a 32-bit number1 between 0 and 4,294,967,295 (about 4 billion) and IPv6 assigns them a 128-bit number2 between 0 and 340,282,366,920,938,463,463,374,607,431,768,211,455 (about 300 duodecillion, a number so large that many dictionaries do not even recognize it as a word). These numbers, called IP addresses, are generally not used directly: humans prefer to use hostnames with DNS providing the translation between them.
IP does not guarantee delivery of messages, leading to reliably protocols like TCP.
internet service provider, a company that allows subscribers to connect to their network and through their gateway to reach the Internet.
A key piece of an operating system, responsible for managing programs’ access to disks, networks, and other hardware resources.
What kernels can do is defined by the computer hardware, which requires that some operations (those that would talk to hardware directly) only be run in kernel mode and stops any program that is not the kernel which tries to do these operations, switching to the kernel and having the kernel decide what to do with the program’s attempt.
A way of having two parties agree on the key they will use for a symmetric cipher without anyone else knowing what key they picked.
The Diffie-Hellman key exchange is by far the most common key exchange protocol.
Short for Local Area Network, a collection of computers connected to and communicating with one another with some level of trust or central management. The computers connected to the same home network typically form a LAN, as do the computers owned by many companies.
Despite the name, there is not firm requirement that computers on a LAN be in the same physical location. However, they do connect to one another without the interposition of computers that are not part of the LAN; if external computers are needed to route traffic, the result is some form of internet.
A set of code that is designed to be used by other programs. Libraries generally implement components that are common to many applications, ranging from under-the-hood data structures and simple algorithms to broad frameworks that provide visual elements used in many apps.
Many libraries are maintained by a small team of developers as a side project and released for free. This makes them vulnerable to supply chain attacks.
A set of simple actions a processor can take, encoded in a compact binary format. Common machine instruction types include:
Machine instructions can be directly converted to and from assembly. Compilers generate machine instructions from source code, but that operation generally cannot be reversed.
A form of programming that works by
Changes made to a software product after it is complete
and shipped. There are three main reasons to change delivered code:
Data about the (main) data.
For example, in an image file the color of each pixel is the main data; information about the camera that took the image is metadata.
Many file formats store metadata along with the data, and many applications ignore most of the metadata.
A group of computers connected to one another and sending information back and forth.
Networks range from very small LANs that might have only two computers (a router and a user’s computer) through multi-LAN networks owned by ISPs through to the entire Internet.
Sometimes people use network
only to refer to centrally-managed networks and thus omit the Internet from its definition.
A type of machine learning where the training happens separately from and before the function selected during training is used.
Offline learning is easier to design than online learning and thus is the default for many machine learning applications.
Sending an IP packet though multiple proxies to prevent identification of who is communicating with whom.
Tor is the largest and best-known onion routing network.
A type of machine learning where new training data is collected and used to change the function being used after some use of the function.
Online learning is harder to design than offline learning and thus is generally limited to cases where the application needs to start acting before it has all the training data, such as when it needs to learn from the outcomes of earlier decisions it has made.
A collection of programs including a kernel and supporting software designed to make other programs easier to build and more consistent in how they behave.
Four of the most popular operating systems (Windows, MacOS, Android, iOS) are very large systems with hundreds of programs and libraries that impose specific designs on how programs should look. Almost all other operating systems (Linux, FreeBSD, Unix, OpenBSD, Irix, Firefly, BeOS, Dos, OS/2, and so on) include a kernel and basic programming libraries but allow windowing environments that support different looks and operational modalities to be run as separate programs. Having a separate windowing environment makes these particularly well suited to web servers and other computers that are not primarily used to run graphical programs for a single user and thus don’t need a windowing environment to be installed at all.
A common way that machine learning might fail, caused by the function family having so many parameters that the function selected matches the details of the specific training data used rather than the underlying patterns the training data was supposed to exemplify.
The most common safeguard against overfitting is to ensure there are many fewer parameters than there are training data. Other techniques pick function families that are not flexible enough to conform to the training data’s details or add mathematical criteria to the training process that detects and avoids parameters that appear to be overfitting.
Most numbers in computers are represented in a fixed number of bits. If math would produce a number that requires more bits than that, the number overflows. What happens after overflow depends on the kind of number:
For integers, the most common is to simply discard the digits that won’t fit; so 10011 stored in 4 digits would become 0011.
For floats, when the exponent overflows the number changes into a special infinity
value. It loses its ability to engage in most math, but is still clearly a big number
.
A placeholder, often implemented as a variable, to achieve generalization.
An algorithm to compute has no parameters: each time it is used it creates the same result. An algorithm to compute has one parameter, : to use the algorithm we have to provide an argument, a value to use in place of for this run of the algorithm; and for each we provide the algorithm will create a different result.
A message that has not been encrypted, or has been decrypted.
Plaintext
is often used with ciphertext
to discuss encryption processes, such as the contents of webpages. It is also used to refer to messages that are never encrypted, such as most email.
A method of training an AI system in several steps.
The pretraining step typically uses as much training data as possible to learn broad patterns without overfitting.
The fine-tuning step moves the function selected during pretraining closer to a smaller, more desirable subset of the training data.
Fine-tuning can be repeated to further adjust the learned function.
Often used as it is in English, to refer to anything that’s not going well or needs to be addressed.
Also used as a desired the relationship between inputs and outputs defining the goal of an algorithm. Find if a number is odd
is a problem; write it in binary and see if its low-order bit is 1
is one of many algorithms for solving that problem.
An algorithm solving a problem needs to pick what data to use to represent the input and output information. An implementation of the algorithm as a function will have the input data as its parameters.
One of the key qualifiers of a computer, indicating that the specific computations it performs is not fixed when it is created but rather can be specified as inputs to the computer.
Usually, programmable
is also assumed to mean that the program it follows is stored inside the computer’s memory and can be an entire algorithm, not just one of a machine-defined set of operations selected by simple switches and buttons.
miningbecause successful blocks are rewarded with additional digital currency.
Changing how code operates without changing what it does.
There are two common reasons to refactor code.
When adding a new feature, refactoring can be used to first update the design and implementation to make the new feature simpler to add.
When dealing with code that has accumulated technical debt, refactoring can be used to try to simplify the code and make it less brittle.
A family of AI tasks that predict one or more values based on other values. This is continuous (the categorical version is called classification
) and supervised (because the training data has the value that should be predicted.
Regression is an older topic than AI and is widely studied in statistics.
A component of programs that indicates that some set of actions should be performed repeatedly. Also called iteration or looping.
The most common programming language vocabulary for repetition are for and while.
What the client or user of software wants from the software. This can be divided into two broad types:
The process of defining the requirements of a project.
Requirements are often ambiguous and poorly defined initially, and often conflict, and often conflict in non-obvious ways (for example, the nonfunctional requirements that the code be high quality, inexpensive, and delivered cannot all be satisfied). Requirements elicitation is generally an interactive process with the developer asking questions and posing possible solutions and the clients commenting on them and adding priorities to resolve conflicts.
There are two types of selection.
One, also called conditionals, jumps, or branches, is a component of a program that indicates that some set of actions should only be performed under certain conditions and skipped if those conditions do not hold. The most common programming language vocabulary for this type of selection are if and else.
The other, also called indexing, is a component of a program that indicates which one out of several candidate pieces of data should be used. The most common programming language vocabulary for this type of selection are the name of the set of candidates, followed by which candidate in the set to chose in brackets, like some_list[3] to pick item 3 out of a list.
A way of attesting approval of a document, using a special type of encryption that has two keys: one to encrypt, another to decrypt. The encryption key is kept private, while the decryption key is shaded publicly.
To create a signature, the signer hashes the document they wish to sign and encrypts the hash.
To check a signature, the checker both hashes the document and decrypts the signature; if those two operations result in the same hash, the signature is valid.
Signatures are widely used as part of authentication, for example in the creation of certificates, as well as to ensure message integrity.
A set of related repeated or iterative organizations of software development. Spiral models are usually group development work into four main pieces:
The spiral model is sometimes offered as an alternative to the waterfall model, sometimes posed as having the entire waterfall model in step 3 above, and sometimes posed with one pass around the loop for each step of the waterfall model.
A more specific domain. For example, a.b.c is a subdomain of b.c.
Because DNS is hierarchical, the owner of a domain can create as many subdomains as they wish by ensuring the computer identified by their IP address responds to DNS requests. No special rights or permissions are needed beyond owning the domain.
One form of cyber crime, based on inserting malicious code into the upstream libraries used by the software that is being attacked.
The common approach to these attacks runs as follows:
The SI abbreviation for tera
meaning one trillion (T) followed by the abbreviation for Byte (B). Commonly used to measure the disk space of computers, indicating they can store a trillion bytes of data.
Some of that space is typically used to store an index of sorts to help find the rest of the data, so the usable space is generally lower than the advertised, theoretically-provided space.
The accumulated poor design created by modifying code after it is designed, often as a result of maintenance but sometimes as a result of the initial design being inadequate.
Technical debt makes code brittle.
Refactoring is often used to paying down technical debt. If too much technical debt accumulates, it becomes cheaper to start over than to continue maintaining the existing software.
Technical debt often accumulates because developers are pressured to get the next thing out now, even if that means cutting corners and not keeping the code clean and well-designed. Like financial debt, technical debt gives us things we want now, but with increased cost later on.
The part of software development that attempts to check if the software works.
Most tests are code that run other code to see if it does the right thing. Some tests are done by humans using the software, and are called alpha tests if those humans are internal to the development process or beta tests if they are external.
Tests do not prove that software works; they just offer some confidence that it seems to work. It is possible to prove that software works, but doing so typically takes at least 10× as much time and money as creating the software itself, and thus is rarely seen as worth the investment. As a consequence, most software ships with bugs, which is one of the reasons that maintenance is necessary.
Short for transmission control protocol
, a tool for adding reliability on top of an unreliable communication medium like IP. There are several key ideas used by TCP:
Because the acknowledgments roughly double the number of messages sent, some protocols that depend on low latency chose not to use TCP, instead having their own way of handling dropped packets.
A large volunteer-run group of computers participating in onion routing.
The computers that make up Tor are called relays. Each relay accepts incoming encrypted messages that when decrypted contain a destination computer and a message to send them and forwards the message to that computer.
To send a message via Tor, a client picks three relays and sends
Tor is used for many purposes, including hosting the dark web.
Example data that an algorithm solving a problem should be able to process.
For supervised learning, training data pairs each input with an expected output and the quality of the function is determined by how close the function’s output is to that input.
For unsupervised learning, training data is just input and some other property of output (typically how the outputs on different inputs compare) determined the quality of the function.
How the bits inside some piece of data are converted into meaning.
For example, the bits 01100110 could have many meanings. If they are interpreted with the data type integer
they mean 102. If they are interpreted with the data type UTF-8 character
they mean lower-case F (f). The bits (the information we actually store in the computer) have not changed, only how we interpret it.
Often used to mean the libraries and other resources used by a piece of software.
Sometimes used as a direction: software uses libraries that are upstream from it and is used by those that are downstream from it.
Short for universal resource locator,
a human-readable way of identifying a computer and the resource on the computer to access. URLs generally have five components:
A scheme, like https, which indicates what data format will be sent.
The scheme is followed by ://
A hostname, like illinois.edu, which is turned into the IP address of a specific computer by DNS.
It is possible, but quite unusual, for the hostname to be preceded by a username and password separated by a color : and followed by an at sign @.
It is possible, but uncommon, for the hostname to be followed by a colon : and a port number, which identifies which program on the host computer should handle the request.
A path, which starts with a slash / and is intended to name a resource on that computer.
Optionally, a query, which starts with a question mark / and is intended to store the arguments to be sent to the resource, assuming it is a program and not a file.
Not all computers use queries for arguments they way they were designed, but most do.
Optionally, a fragment or hash, which starts with an number sign/hash/ocotothorp #. The fragment is not sent to the computer identified by the hostname at all; instead, it is used by browsers to pick a part of the page to scroll into view.
The name of a location where data can be stored.
Unlike in mathematics, variables in computing are expected to change their meaning as a program progresses. It is perfectly OK for a program to say x = 3 on one line and x = 4 on another.
Most programming languages use = not to mean is the same as
but rather gets the value of
, in a directional way. x = 3 + 4 means the memory named by variable x should have the number 7 stored inside it. Conversely, 3 + 4 = x is a nonsensical an erroneous request that the number 7 have the contents of the memory named by variable x put inside it.
A heavier-weight way of isolating programs and providing the illusion of each running on its own computer than a container.
In a virtual, each system call is routed not to the usual kernel but to a second virtual kernel that only exists within the virtual machine. When that guest kernel tries to talk to hardware, that effort is interrupted by the real kernel and adjusted to prevent the guest kernel from interacting with anything else the computer is running.
Virtual machines are sometimes used in cloud-based computing, but containers are more popular. VPN
Short for virtual private network
, a computer that agrees to forward messages for other computers in a more private way than is provided by IP.
Using IP, even encrypted messages have the sending and receiving IP address visible to other computers making up the Internet. VPNs accept messages over IP from their clients which are encrypted versions of messages to send to others, send them to those recipients, receive the replies, and send the encrypted replies back to the the clients.
Even more private communication is provided by onion routing, which can be provided by paying for multiple VPNs or through volunteer networks like Tor.
Some aspect of a computer system – usually software, but sometimes hardware – that has the potential to be used in ways that the designers did not intend and the legitimate users do not desire.
Vulnerabilities can arise from many sources: programmer error, software designer oversight, unexpected interplay between individually-correct components, failure to remove development aids like bypass codes, malicious insertion by software developers with secret agendas, and so on.
It is generally assumed that nearly all computer systems contain many vulnerabilities, and new vulnerabilities are discovered every day. It is essential that software be regularly updated to close vulnerabilities as they are discovered to prevent them being targeted by exploits.
Any of several ways of breaking software development into a sequence of steps, one undertaken after another. One common breakdown is: