Introduction to Data Communications
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red-triangle54. Internet Protocol

The Network Layer protocol for TCP/IP is the Internet Protocol (IP). It uses IP addresses and the subnet mask to determine whether the datagram is on the local or a remote network. If it is on the remote network, the datagram is forwarded to the default gateway which is a router that links to another network.

IP keeps track of the number of transverses through each router that the datagram goes through to reach its destination. Each transvers is called a hop. If the hop count exceeds 255 hops, the datagram is removed and the destination considered unreachable. IP's name for the hop count is called Time To Live (TTL).

ballIP Addresses

IP addresses consist of a 32 bit number and is represented by the dot-decimal format. for example: 142.110.237.1 is an IP address. There are 4 decimal digits separated by three dots. Each digit is allowed the range of 0 to 255 which corresponds to 8 bits (one byte) of information.

A portion of an IP address represents the network address and the remaining portion the host address. For example: 142.110.237.1 is the IP address of a firewall. The network that the firewall resided on is 142.110.237.0 (Note: IP addresses that end in a 0 represent network addresses). The host address of the firewall is 0.0.0.1 (Note: the network portion of the IP address is represented by 0s). Each host on the network and Internet must have a unique IP address. There are ways around having each host a unique IP address and they are discussed under firewalls.

The Network Information Center (NIC) assigns network addresses to the Internet. You must apply to receive a IP network address. Depending on the class (more on this later) of the IP address, you can then assign as many host IP addresses as allowed.

An alternative is to "rent" IP addresses from your local Internet Service Provider (ISP). They usually own the rights to a block of IP addresses and will rent them out for a fee.

Introduction to Data Communications
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