Nortel BCM50a - Subnetting with Class A and Class B networks.

238 Appendix F IP subnetting
N0115791
Table 54 is a summary for class C subnet planning.
Subnetting with Class A and Class B networks.
For class A and class B addresses the subnet mask also determines which bits are
part of the network number and which are part of the host ID.
A class B address has two host ID octets available for subnetting and a class A
address has three host ID octets (see Table 43) available for subnetting.
Table 55 is a summary for class B subnet planning.
7 192 193 222 223
8 224 225 254 255
Table 54 Class C subnet planning
No. Borrowed Host Bits Subnet Mask No. Subnets No. Hosts per Subnet
1 255.255.255.128 (/25) 2 126
2 255.255.255.192 (/26) 4 62
3 255.255.255.224 (/27) 8 30
4 255.255.255.240 (/28) 16 14
5 255.255.255.248 (/29) 32 6
6 255.255.255.252 (/30) 64 2
7 255.255.255.254 (/31) 128 1
Table 55 Class B subnet planning
No. “Borrowed” Host Bits Subnet Mask No. Subnets No. Hosts per Subnet
1 255.255.128.0 (/17) 2 32 766
2 255.255.192.0 (/18) 4 16 382
3 255.255.224.0 (/19) 8 8190
4 255.255.240.0 (/20) 16 4094
Table 53 Eight subnets
Subnet Subnet Address First Address Last Address Broadcast Address