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A-3
Cisco ASA 5500 Series Configuration Guide using ASDM
Appendix A 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 A-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 A-3
• Class B-Size Network Address, page A-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 A-2 shows the 8-host subnets (/29) of 192.168.0.x.
Table A-1 Hosts, Bits, and Dotted-Decimal Masks
Hosts
1
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
Table A-2 Class C-Size Network Address
Subnet with Mask /29 (255.255.255.248) Address Range
1
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