The Three Classes of IP Addresses Used in Networks Today

TABLE 2 . 1
The Three Classes of IP Addresses Used in Networks Today
Class
Leading Bit
Pattern
Default
Subnet Mask Address Range
Number of Addresses
per Network
A 0 255.0.0.0 1.0.0.0�"126.255.255.255 16,777,214
B 10 255.255.0.0 128.0.0.0�"191.255.255.255 65,534
C 110 255.255.255.0 192.0.0.0�"223.255.255.255 254
D 1110 None 224.0.0.0�"239.255.255.255 Multicast
E 1111 None 240.0.0.0�"255.255.255.255 Experimental
To ensure efficient routing, Internet designers defined a mandate for the leading bits section of
the address for each network class. For example, because a router knows that a Class A network
address always starts with 0, the router can determine the default boundary between network and
host portions after reading only the first bit of its address. This capability is invaluable when the
router has no way of determining the actual prefix length of an address. It allows at least a minimum
level of functionality and reachability.