Networking

OSI Model

OSI Model The OSI (Open Systems Interconnection) model was created by the ISO to help standardize communication between computer systems. It divides communications into seven different layers, which each include multiple hardware standards, protocols, or other types of services. The seven layers of the OSI model include: The Physical layer The Data Link layer The Network layer The Transport layer The Session layer The Presentation layer The Application layer When one computer system communicates with another, whether it is over a local network or the Internet, data travels through these seven layers. It begins with the physical layer of the transmitting system and travels through the other layers to the application layer. Once the data reaches the application layer, it is processed by the receiving system. In some cases, the data will move through the layers in reverse to the physical layer of the receiving computer. The best way to explain how the OSI model works is to use a real life example. In the following illustration, a computer is using a wireless connection to access a secure website. The communications stack begins with the (1) physical layer. This may be the computer's Wi-Fi card, which transmits data using the IEEE 802.11n standard. Next, the (2) data link layer might involve connecting to a router via DHCP. This would provide the system with an IP address, which is part of the (3) network layer. Once the computer has an IP address, it can connect to the Internet via the TCP protocol, which is the (4) transport layer. The system may then establish a NetBIOS session, which creates the (5) session layer. If a secure connection is established, the (6) presentation layer may involve an SSL connection. Finally, the (7) application layer consists of the HTTP connection to the website. The OSI model provides a helpful overview of the way computer systems communicate with each other. Software developers often use this model when writing software that requires networking or Internet support. Instead of recreating the communications stack from scratch, software developers only need to include functions for the specific OSI layer(s) their programs use.

Router

Router This is a hardware device that routes data (hence the name) from a local area network (LAN) to another network connection. A router acts like a coin sorting machine, allowing only authorized machines to connect to other computer systems. Most routers also keep log files about the local network activity.

Bandwidth

Bandwidth Bandwidth describes the maximum data transfer rate of a network or Internet connection. It measures how much data can be sent over a specific connection in a given amount of time. For example, a gigabit Ethernet connection has a bandwidth of 1,000 Mbps (125 megabytes per second). An Internet connection via cable modem may provide 25 Mbps of bandwidth. While bandwidth is used to describe network speeds, it does not measure how fast bits of data move from one location to another. Since data packets travel over electronic or fiber optic cables, the speed of each bit transferred is negligible. Instead, bandwidth measures how much data can flow through a specific connection at one time. When visualizing bandwidth, it may help to think of a network connection as a tube and each bit of data as a grain of sand. If you pour a large amount of sand into a skinny tube, it will take a long time for the sand to flow through it. If you pour the same amount of sand through a wide tube, the sand will finish flowing through the tube much faster. Similarly, a download will finish much faster when you have a high-bandwidth connection rather than a low-bandwidth connection. Data often flows over multiple network connections, which means the connection with the smallest bandwidth acts as a bottleneck. Generally, the Internet backbone and connections between servers have the most bandwidth, so they rarely serve as bottlenecks. Instead, the most common Internet bottleneck is your connection to your ISP. NOTE: Bandwidth also refers to a range of frequencies used to transmit a signal. This type of bandwidth is measured in hertz and is often referenced in signal processing applications.

broadband connection

Broadband This refers to high-speed data transmission in which a single cable can carry a large amount of data at once. The most common types of Internet broadband connections are cable modems (which use the same connection as cable TV) and DSL modems (which use your existing phone line). Because of its multiple channel capacity, broadband has started to replace baseband, the single-channel technology originally used in most computer networks. So now when you see companies like AT&T and SBC pushing those fancy "broadband" ads in your face, you'll at least know what they are talking about.

DHCP (Dynamic Host Configuration Protocol)

DHCP Stands for "Dynamic Host Configuration Protocol." DHCP is a protocol that automatically assigns a unique IP address to each device that connects to a network. With DHCP, there is no need to manually assign IP addresses to new devices. Therefore, no user configuration is necessary to connect to a DCHP-based network. Because of its ease of use and widespread support, DHCP is the default protocol used by most routers and networking equipment. When you connect to a network, your device is considered a client and the router is the server. In order to successfully connect to a network via DHCP, the following steps must take place. When a client detects it has connected to a DHCP server, it sends a DHCPDISCOVER request. The router either receives the request or redirects it to the appropriate DHCP server. If the server accepts the new device, it will send a DHCPOFFER message back to the client, which contains the client device's MAC address and the IP address being offered. The client returns a DHCPREQUEST message to the server, confirming it will use the IP address. Finally, the server responds with a DHCPACK acknowledgement message that confirms the client has been given access (or a "lease") for a certain amount of time. DHCP works in the background when you connect to a network, so you will rarely see any of the above steps happen. The time it takes to connect via DHCP depends on the type of router and the size of the network, but it usually takes around three to ten seconds. DHCP works the same way for both wired and wireless connections, which means desktop computers, tablets, and smartphones can all connect to a DHCP-based network at the same time.

DNS (Domain Name System)

DNS Stands for "Domain Name System." Domain names serve as memorizable names for websites and other services on the Internet. However, computers access Internet devices by their IP addresses. DNS translates domain names into IP addresses, allowing you to access an Internet location by its domain name. Thanks to DNS, you can visit a website by typing in the domain name rather than the IP address. For example, to visit the Tech Terms Computer Dictionary, you can simply type "techterms.com" in the address bar of your web browser rather than the IP address (67.43.14.98). It also simplifies email addresses, since DNS translates the domain name (following the "@" symbol) to the appropriate IP address. To understand how DNS works, you can think of it like the contacts app on your smartphone. When you call a friend, you simply select his or her name from a list. The phone does not actually call the person by name, it calls the person's phone number. DNS works the same way by associating a unique IP address with each domain name. Unlike your address book, the DNS translation table is not stored in a single location. Instead, the data is stored on millions of servers around the world. When a domain name is registered, it must be assigned at least two nameservers (which can be edited through the domain name registrar at any time). The nameserver addresses point to a server that has a directory of domain names and their associated IP addresses. When a computer accesses a website over the Internet, it locates the corresponding nameserver and gets the correct IP address for the website. Since DNS translation creates additional overhead when connecting to websites, ISPs cache DNS records and host the data locally. Once the IP address of a domain name is cached, an ISP can automatically direct subsequent requests to the appropriate IP address. This works great until an IP address changes, in which case the request may be sent to the wrong server or the server will not respond at all. Therefore, DNS caches are updated regularly, usually somewhere between a few hours and a few days.

DSL

DSL Stands for "Digital Subscriber Line." DSL is a communications medium used to transfer digital signals over standard telephone lines. Along with cable Internet, DSL is one of the most popular ways ISPs provide broadband Internet access. When you make a telephone call using a landline, the voice signal is transmitted using low frequencies from 0 Hz to 4 kHz. This range, called the "voiceband," only uses a small part of the frequency range supported by copper phone lines. Therefore, DSL makes use of the higher frequencies to transmit digital signals, in the range of 25 kHz to 1.5 MHz. While these frequencies are higher than the highest audible frequency (20 kHz), then can still cause interference during phone conversations. Therefore, DSL filters or splitters are used to make sure the high frequencies do not interfere with phone calls. Symmetric DSL (SDSL) splits the upstream and downstream frequencies evenly, providing equal speeds for both sending and receiving data. However, since most users download more data than they upload, ISPs typically offer asymmetric DSL (ADSL) service. ADSL provides a wider frequency range for downstream transfers, which offers several times faster downstream speeds. For example, an SDSL connection may provide 2 Mbps upstream and downstream, while an ASDL connection may offer 20 Mbps downstream and 1.5 Mbps upstream. In order to access the Internet using DSL, you must connect to a DSL Internet service provider (ISP). The ISP will provide you with a DSL modem, which you can connect to either a router or a computer. Some DSL modems now have built-in wireless routers, which allows you to connect to your DSL modem via Wi-Fi. A DSL kit may also include a splitter and filters that you can connect to landline phones. NOTE: Since DSL signals have a limited range, you must live within a specific distance of an ISP in order to be eligible for DSL Internet service. While most urban locations now have access to DSL, it is not available in many rural areas.

IP address

IP Address An IP address, or simply an "IP," is a unique address that identifies a device on the Internet or a local network. It allows a system to be recognized by other systems connected via the Internet protocol. There are two primary types of IP address formats used today — IPv4 and IPv6. IPv4 An IPv4 address consist of four sets of numbers from 0 to 255, separated by three dots. For example, the IP address of TechTerms.com is 67.43.14.98. This number is used to identify the TechTerms website on the Internet. When you visit http://techterms.com in your web browser, the DNS system automatically translates the domain name "techterms.com" to the IP address "67.43.14.98." There are three classes of IPv4 address sets that can be registered through the InterNIC. The smallest is Class C, which consists of 256 IP addresses (e.g. 123.123.123.xxx — where xxx is 0 to 255). The next largest is Class B, which contains 65,536 IP addresses (e.g. 123.123.xxx.xxx). The largest block is Class A, which contains 16,777,216 IP addresses (e.g. 123.xxx.xxx.xxx). The total number of IPv4 addresses ranges from 000.000.000.000 to 255.255.255.255. Because 256 = 28, there are 28 x 4 or 4,294,967,296 possible IP addresses. While this may seem like a large number, it is no longer enough to cover all the devices connected to the Internet around the world. Therefore, many devices now use IPv6 addresses. IPv6 The IPv6 address format is much different than the IPv4 format. It contains eight sets of four hexadecimal digits and uses colons to separate each block. An example of an IPv6 address is: 2602:0445:0000:0000:a93e:5ca7:81e2:5f9d. There are 3.4 x 1038 or 340 undecillion) possible IPv6 addresses, meaning we shouldn't run out of IPv6 addresses anytime soon.

ISP (Internet Service Provider)

ISP Stands for "Internet Service Provider." An ISP provides access to the Internet. Whether you're at home or work, each time you connect to the Internet, your connection is routed through an ISP. Early ISPs provided Internet access through dial-up modems. This type of connection took place over regular phone lines and was limited to 56 Kbps. In the late 1990s, ISPs began offering faster broadband Internet access via DSL and cable modems. Some ISPs now offer high-speed fiber connections, which provide Internet access through fiber optic cables. Companies like Comcast and Time Warner provide cable connections while companies like AT&T and Verizon provide DSL Internet access. To connect to an ISP, you need a modem and an active account. When you connect a modem to the telephone or cable outlet in your house, it communicates with your ISP. The ISP verifies your account and assigns your modem an IP address. Once you have an IP address, you are connected to the Internet. You can use a router (which may a separate device or built into the modem) to connect multiple devices to the Internet. Since each device is routed through the same modem, they will all share the same public IP address assigned by the ISP. ISPs act as hubs on the Internet since they are often connected directly to the Internet backbone. Because of the large amount of traffic ISPs handle, they require high bandwidth connections to the Internet. In order to offer faster speeds to customers, ISPs must add more bandwidth to their backbone connection in order to prevent bottlenecks. This can be done by upgrading existing lines or adding new ones.