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Chapter 5. Configuring IP
The Internet Protocol (IP) is a packet-based protocol used to exchange data over computer networks. It is
the foundation on which all other IP protocols are built. IP is a network-layer protocol that contains
addressing and control information that allows data packets to be routed.
This section describes how to configure the Internet Protocol (IP).
Configuring IP Addressing
A number of tasks are associated with configuring IP. A basic and required task for configuring IP is to
assign IP addresses to network interfaces. Doing so enables the interfaces and allows communication with
hosts on those interfaces using IP. Associated with this task are decisions about subnetting and masking the
IP addresses.
5.1 Assign IP Addresses to Network Interfaces
An IP address is a location to and from which IP datagrams can be sent. IP addresses were traditionally
divided into three classes. The Class A Internet address format allocated the highest eight bits to the
network field and set the highest-order bit to 0 (zero). The remaining 24 bits formed the host field. The Class
B Internet address allocated the highest 16 bits to the network field and set the two highest-order bits to 1, 0.
The remaining 16 bits formed the host field. The Class C Internet address allocated the highest 24 bits to the
network field and set the three highest-order bits to 1,1,0. The remaining eight bits formed the host field.
The table below lists the traditional classes and ranges of IP addresses, and their status.
Class Address or Range Status
A 0.0.0.0
1.0.0.0 to 126.0.0.0
127.0.0.0
Reserved
Available
Reserved
B 128.0.0.0 to 191.254.0.0
191.255.0.0
Available
Reserved
C 192.0.0.0
192.0.1.0 to 223.255.254
223.255.255.0
Reserved
Available
Reserved
D 224.0.0.0 to 239.255.255.255 Multicast group addresses
E 240.0.0.0 to 255.255.255.254
255.255.255.255
Reserved
Broadcast
With the rapid expansion of networks being connected to the Internet, critical problems were seen with the
traditional classified addressing scheme. It was possible that IP addresses would run out, and routing tables
would be overwhelmed. Thus, the Classless Inter-Domain Routing (CIDR) addressing scheme was created.
CIDR replaces the older process of assigning IP addresses with general prefixes of 8, 16, or 24 bits. CIDR
uses prefixes of 13 to 27 bits. A CIDR address includes the standard 32-bit IP address and adds information
on how many bits are used for the network prefix. In the IP address 206.203.1.35/27, the “/27” indicates that
the first 27 bits are used to identify the unique network, and the remaining bits are used to identify the
specific host. Now, blocks of addresses can be better fitted to even very small or very large networks. The
following table describes the Class C equivalent of CIDR prefixes.