B-3
Cisco ASA 5500 Series Configuration Guide using the CLI
AppendixB Addresses, Protocols, and Ports
IPv4 Addresses and Subnet Masks
Determining the Subnet Mask
To determine the subnet mask based on how many hosts you want, see Table B- 1.
Determining the Address to Use with the Subnet Mask
The following sections describe how to determine the network address to use with a subnet mask for a
Class C-size and a Class B-size network. This section includes the following topics:
Class C-Size Network Address, page B-3
Class B-Size Network Address, page B-4

Class C-Size Network Address

For a network between 2 and 254 hosts, the fourth octet falls on a multiple of the number of host
addresses, starting with 0. For example, Table B -2 shows the 8-host subnets (/29) of 192.168.0.x.
TableB-1 Hosts, Bits, and Dotted-Decimal Masks
Hosts1
1. The first and last number of a subnet are reserved, except for /32, which identifies a single host.
/Bits Mask Dotted-Decimal Mask
16,777,216 /8 255.0.0.0 Class A Network
65,536 /16 255.255.0.0 Class B Network
32,768 /17 255.255.128.0
16,384 /18 255.255.192.0
8192 /19 255.255.224.0
4096 /20 255.255.240.0
2048 /21 255.255.248.0
1024 /22 255.255.252.0
512 /23 255.255.254.0
256 /24 255.255.255.0 Class C Network
128 /25 255.255.255.128
64 /26 255.255.255.192
32 /27 255.255.255.224
16 /28 255.255.255.240
8 /29 255.255.255.248
4 /30 255.255.255.252
Do not use /31 255.255.255.254
1 /32 255.255.255.255 Single Host Address
TableB-2 Class C-Size Network Address
Subnet with Mask /29 (255.255.255.248) Address Range1
192.168.0.0 192.168.0.0 to 192.168.0.7
192.168.0.8 192.168.0.8 to 192.168.0.15