Nortel BCM50a, Configuration Manual

Page: 233 / 315

Appendix F IP subnetting 233
BCM50a Integrated Router Configuration — Advanced
Subnet masks
A subnet mask is used to determine which bits are part of the network number,
and which bits are part of the host ID (using a logical AND operation). A subnet
mask contains 32 bits. If there is a 1 in the bit, then the corresponding bit of the IP
address is part of the network number. If a bit in the subnet mask is 0 then the
corresponding bit in the IP address is part of the host ID.
Subnet masks are expressed in dotted decimal notation just as IP addresses are.
The natural masks for class A, B, and C IP addresses are as follows.
Subnetting
With subnetting, the class arrangement of an IP address is ignored. For example, a
class C address no longer has to have 24 bits of network number and 8 bits of host
ID. With subnetting, some of the host ID bits are converted into network number
bits. By convention, subnet masks always consist of a continuous sequence of
ones beginning from the left most bit of the mask, followed by a continuous
sequence of zeros, for a total number of 32 bits.
Since the mask is always a continuous number of ones beginning from the left,
followed by a continuous number of zeros for the remainder of the 32-bit mask,
you can simply specify the number of ones instead of writing the value of each
octet. This is usually specified by writing a / followed by the number of bits in the
mask after the address.
For example, 192.1.1.0 /25 is equivalent to saying 192.1.1.0 with mask
255.255.255.128.
Table 45 Natural Masks
Class Natural mask
A 255.0.0.0
B 255.255.0.0
C 255.255.255.0