The three most commonly used classes of TCP/IP networks ...



IP Addresses

The three most commonly used classes of TCP/IP networks are: Class A, Class B, and Class C. Each class uses a different address format to identify subnets and devices within it.

An IP address identifies each device (or host) on the network. It is important that each device has a unique IP address. When two devices have the same IP address, neither one can access the network, and, in addition, they can also prevent other devices from using the network.

Generally, an IP address has the following format:

nnn.nnn.nnn.nnn

where nnn are numbers between 0 and 255. Each group of numbers separated by periods is called an octet.

The IP address has two separate components: a subnet address (or network number) and a host address (or host number). All computers on the same subnet share a network number but have different host numbers. Certain numbers are reserved and cannot be assigned as host IDs. For example, 0 and 255 are reserved on a Class "C" network.

The Internet Assigned Numbers Authority (IANA) assigns subnet addresses—the first part of the IP address—for networks that are directly linked to the Internet. Because the IP address for a computer (or any network device) needs to be unique, host addresses are typically assigned automatically using DHCP. The network administrator is usually responsible for IP address assignment on a TCP/IP network.

A Class C IP address has the following format:

nnn.nnn.nnn.hhh

where nnn and hhh are numbers between 0 and 255 (referred to as octets).

In a Class C address, the first three octets represent the subnet address and must be between 192.1.1 and 223.254.254. The fourth octet identifies a specific system, or host, on a network. (0 and 255 are reserved values and cannot be used for host numbers). For example, in the IP address 192.168.45.2, the first three octets (192.168.45) identify the network, and the last octet (2) identifies a computer system (or network device). Other systems on the same network could have IP addresses such as 192.168.45.3 and 192.168.45.212.

A Class B IP address has the following format:

nnn.nnn.nnn.hhh

where nnn and hhh are numbers between 0 and 255 (referred to as octets). In a Class B address, the first two octets represent the subnet address and must be between 128.1 and 191.254. The remaining octets identify a specific system, or host, on a network.

A Class A IP address has the following format:

nnn.nnn.nnn.hhh

where nnn and hhh are numbers between 0 and 255 (referred to as octets). In a Class A address, the first octet represents the subnet address and must be between 1 and 126. The remaining octets identify a specific system, or host, on a network.

IP Subnet Masks

The subnet mask indicates which portion of the IP address is used to identify the local network or subnet.

A standard Class C network has a subnet mask of 255.255.255.0. This mask indicates that the first three octets are used to identify the subnet, and the last octet is used for the host number. The number of IP addresses you can assign to hosts on your network is called the IP address space. The address space of a Class C network is 254. Both Class A networks (subnet mask 255.0.0.0) and Class B networks (subnet mask 255.255.0.0) have much larger IP address spaces.

TCP/IP Default Gateway (Router)

A gateway (or router) is a system that connects a network with one or more other networks. A gateway can be a dedicated network device, or a standard computer with multiple network interface cards.

In a TCP/IP network that has multiple subnets, gateways forward messages from one subnet to another. When a host needs to transmit a message to another host on a different subnet, it sends the message to a gateway. The gateway routes the message to its final destination. Without a gateway, a host cannot communicate to other hosts on other networks.

Most TCP/IP networks have at least one gateway. When you configure a network device and you want it to be able to communicate with other devices outside of its subnet, you need to identify at least one TCP/IP gateway (known as the default gateway). You identify the gateway by its IP address.

Automatic IP Address Assignment (DHCP)

DHCP lets individual computers lease an IP address for some predefined amount of time (specified in the DHCP server configuration). If a system disconnects from the network for a short period of time, it usually is reassigned the same IP address when it reconnects. If it disconnects for an extended period of time, it may be assigned a different IP address. If you are using DHCP to assign an IP address to your server, this assignment is likely to remain the same, unless you turn the server off overnight or during the weekend. To make sure that the IP address never changes, assign a fixed address to your server.

Ports

Generally speaking, a computer has a single physical connection to the network. All data destined for a particular computer arrives through that connection. However, the data may be intended for different applications running on the computer. So how does the computer know which application to forward data to? Through the use of ports.

Data transmitted over the Internet is accompanied by addressing information that identifies the computer and the port that it's destined for. The computer is identified by its 32-bit IP address, which IP uses to deliver data to the right computer on the network. Ports are identified by a 16-bit number, which TCP and UDP use to deliver the data to the right application.

In connection-based communication, an application establishes a connection with another application by binding a socket to a port number. This has the effect of registering the application with the system to recieve all data destined for that port. No two applications can bind to the same port: Attempts to bind to a port that is already in use will fail.

In datagram-based communication, the datagram packet contains the port number of its destination.

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Definition: The TCP and UDP protocols use ports to map incoming data to a particular process running on a computer.

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Port numbers range from 0 to 65535 (because ports are represented by 16-bit numbers). The port numbers ranging from 0 - 1023 are restricted--they are reserved for use by well-known services such as HTTP and ftp and other system services. Your applications should not attempt to bind to these ports. Ports that are reserved for well-known services such as HTTP and ftp are called well-known ports.

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