NETGEAR FVS124G manual Netmask Notation Translation Table for One Octet, Netmask Formats

Reference Manual for the ProSafe VPN Firewall 25 with 4 Gigabit LAN and Dual WAN Ports

Although the preceding example uses the entire third octet for a subnet address, note that you are not restricted to octet boundaries in subnetting. To create more network numbers, you need only shift some bits from the host address to the network address. For instance, to partition a Class C network number (192.68.135.0) into two, you shift one bit from the host address to the network address. The new netmask (or subnet mask) is 255.255.255.128. The first subnet has network number 192.68.135.0 with hosts 192.68.135.1 to 129.68.135.126, and the second subnet has network number 192.68.135.128 with hosts 192.68.135.129 to 192.68.135.254.

Note: The number 192.68.135.127 is not assigned because it is the broadcast address of the first subnet. The number 192.68.135.128 is not assigned because it is the network address of the second subnet.

The following table lists the additional subnet mask bits in dotted-decimal notation. To use the table, write down the original class netmask and replace the 0 value octets with the dotted-decimal value of the additional subnet bits. For example, to partition your Class C network with subnet mask 255.255.255.0 into 16 subnets (4 bits), the new subnet mask becomes 255.255.255.240.

Table 9-1. Netmask Notation Translation Table for One Octet

Number of Bits Dotted-Decimal Value

1128

2192

3224

4240

5248

6252

7254

8255

The following table displays several common netmask values in both the dotted-decimal and the masklength formats.

Table 9-2. Netmask Formats

Dotted-Decimal Masklength

255.0.0.0/8

255.255.0.0/16

B-6 Network, Routing, Firewall, and Basics

202-10085-01, March 2005