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).
IP addresses consist of a 32 bit number and is represented by the dot-decimal format. for example: 22.214.171.124 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: 126.96.36.199 is the IP address of a firewall. The network that the firewall resided on is 188.8.131.52 (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.
IP Address Classifications
There is a formal structure to the assignment of IP addresses. IP addresses are assigned by the Network Information Center (NIC) who is a central authority with the responsibility of assigning network addresses.
There are several classifications of IP addresses. They include network addresses and special purpose addresses.
Class A Addresses
IP address range 184.108.40.206 to 127.0.0.0
Number of networks available: 125 (see special addresses below)
Number of hosts per network: 16,777,214
Net Mask: 255.0.0.0 (first 8 bits are ones)
Special Addresses: 10.0.0.0 is used for networks not connected to the Internet
127.0.0.0 is the loopback address for testing (see ping)
Class A addresses always have bit 0 set to 0, bits 1-7 are used as the network ID. Bits 8-31 are used as the host ID.
Class A networks are used by very large companies such as IBM, US Dept of Defense and AT&T.
Class B Addresses
IP address range 220.127.116.11 to 18.104.22.168
Number of networks available: 16,382 (see special addresses below)
Number of hosts per network: 65,534
Net Mask: 255.255.0.0 (first 16 bits are ones)
Special Addresses: 172.16.0.0 to 172.31.0.0 are used for networks not
connected to the Internet
Class B addresses always have bit 0 and 1 set to 10, bits 2-15 are used as the network ID. Bits 16-31 are used as the host ID. Class B networks are assigned to large companies and universities.
Class C Addresses
IP address range 192.0.0.0 to 22.214.171.124
Number of networks available: 2,097,150 (see special addresses below)
Number of hosts per network: 254
Net Mask: 255.255.255.0 (first 24 bits are ones)
Special Addresses: 192.168.1.0 to 192.168.255.0are used for networks not
connected to the Internet
Class C addresses always have bits 0-2 set to 110, bits 3-24 are used as the network ID. Bits 25-31 are used as the host ID. Class C network addresses are assigned to small companies and local Internet providers.
Class D Addresses
IP address range 126.96.36.199 to 188.8.131.52
Use: Multicasting addresses
Class E Addresses
IP address range 240.0.0.0 to 255.0.0.0
Use: Reserved by the Internet for its own use.
If you try to ping a Class E address, you should get the error message that says that it is an invalid IP address.
Reserved IP Addresses
The following IP addresses are reserved:
127.0.0.0 Network addresses used for localhost mode (testing IP stack)
255.255.255.255 An IP address consisting of all 1s in binary (255).
x.x.x.0 An IP address with the host portion consisting of 0s.
Used to indicate the network address. Newer routers
have the option of allowing these addresses.
184.108.40.206 - 255.0.0.0 Class D addresses.
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