VLSM and CIDR - Routing Protocols and Concepts - page 247

Table 6-2 Number of Networks and Hosts per Network for Each Class

Address Class First Octet Number of Possible Number of Hosts

Range Networks per Network

Class A 0 to 127 128 (2 are reserved) 16,777,214

Class B 128 to 191 16,384 65,534

Class C 192 to 223 2,097,152 254

Class A networks used the first octet for network assignment, which translated to a 255.0.0.0 classful
subnet mask. Because only 7 bits were left in the first octet (remember, the first bit is always 0), this
made 2 7 , or 128 , networks.

With 24 bits in the host portion, each Class A address had the potential for over 16 million individual
host addresses.

Class B networks used the first two octets for network assignment, which translated to a 255.255.0.0
classful subnet mask. With the first 2 bits already established as 1 and 0, 14 bits remained for assign-
ing networks, which resulted in 16,384 Class B network addresses. Because each Class B network
address contained 16 bits in the host portion, it controlled 65,534 addresses.

Class C networks used the first three octets for network assignment, which translated to a
255.255.255.0 classful subnet mask. With the first 3 bits already established as 1, 1, and 0, 21 bits
remained for assigning networks, for over 2 million Class C networks. But, each Class C network
only had 8 bits in the host portion, or 254 possible host addresses.

Classful Routing Protocol

Remember, a classful routing protocol does not include the subnet mask in the routing update. It could
be determined by the value of the first octet, or more accurately, the first 3 bits of the address.

Classless IP Addressing

In 1993, the Internet Engineering Task Force (IETF) introduced classless interdomain routing ,or
CIDR (RFC 1517 ).

What two major provisions did CIDR allow?

More efficient use of IPv4 address space

■

Prefix aggregation, which reduced the size of routing tables

■

To CIDR-compliant routers, address class is meaningless. The network portion of the address is deter-
mined by the network subnet mask, also known as the network prefix , or prefix length (/8, /19, and so
on). The network address is no longer determined by the class of the address.

CIDR and Route Summarization

CIDR uses variable - length subnet masks (VLSM) to allocate IP addresses to subnets according to
individual need rather than by class. CIDR allowed prefix aggregation, which you already know as
route summarization . Recall from Chapter 2, “Static Routing,” that you can create one static route for
multiple networks. Internet routing tables were now able to benefit from the same type of aggregation
of routes by combining multiple class addresses into one supernet , which summarizes multiple net-
work addresses with a mask less than the classful mask.