The extended network prefix has traditionally been identified by the
subnet mask. For example, if an administrator has the /16 address of and wants to use the entire third octet to represent the subnet
number, the administrator must specify a subnet mask of 
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 with a subnet mask of can also be expressed as 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
U N D E R S TA N D I N G   I P   A D D R E S S I N G
1 0






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