The extended network prefix has traditionally been identified by the

subnet mask. For example, if an administrator has the /16 address of

130.5.0.0 and wants to use the entire third octet to represent the subnet

number, the administrator must specify a subnet mask of 255.255.255.0. 

The bits in the subnet mask and the Internet address have a one to one

correspondence. The bits of the subnet mask are set to 1 (one) if the sys 

tem examining the address should treat the corresponding bit in the IP

address as part of the extended network prefix. The bits in the mask are

set to 0 (zero) if the system should treat the bit as part of the host num 

ber. This numbering is illustrated in Figure 10. 

F I G U R E   1 0 .   S u b n e t   M a s k

The standards describing modern routing protocols often refer to the

extended network prefix length rather than the subnet mask. The pre 

fix length is equal to the number of contiguous one bits in the tradi 

tional subnet mask. This means that specifying the network address

130.5.5.25 with a subnet mask of 255.255.255.0 can also be expressed as

130.5.5.25/24. The / notation is more compact and eas 

ier to understand than writing out the mask in its traditional dotted 

decimal format. This is illustrated in Figure 11. 

F I G U R E   1 1 .   E x t e n d e d   N e t w o r k   P re f i x   L e n g t h

Note that modern routing protocols still carry the subnet mask. None of

the Internet standard routing protocols have a 1 byte field in the header

that contains the number of bits in the extended network prefix. Each

routing protocol is still required to carry the complete four octet subnet

mask. 

U N D E R S TA N D I N G   I P   A D D R E S S I N G

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