ZyXEL Communications G.SHDSL.bis 4-port Security Gateway P-793H - Manual (Page 396)

P-793H User’s Guide
396 Appendix F IP Addresses and Subnetting
The first mask shown is the class “C” natural mask. Normally if no mask is specified it is
understood that the natural mask is being used.
Example: Two Subnets
As an example, you have a class “C” address 192.168.1.0 with subnet mask of 255.255.255.0.
The first three octets of the address make up the network number (class “C”). You want to
have two separate networks.
Divide the network 192.168.1.0 into two separate subnets by converting one of the host ID bits
of the IP address to a network number bit. The “borrowed” host ID bit can be either “0” or “1”
thus giving two subnets; 192.168.1.0 with mask 255.255.255.128 and 192.168.1.128 with
mask 255.255.255.128.
Note: In the following charts, shaded/bolded last octet bit values indicate host ID bits
“borrowed” to form network ID bits. The number of “borrowed” host ID bits determines
the number of subnets you can have. The remaining number of host ID bits (after
“borrowing”) determines the number of hosts you can have on each subnet.
255.255.255.248 /29 1111 1000
255.255.255.252 /30 1111 1100
Table 143 Alternative Subnet Mask Notation (continued)
SUBNET MASK SUBNET MASK “1” BITS LAST OCTET BIT VALUE
Table 144 Two Subnets Example
IP/SUBNET MASK NETWORK NUMBER HOST ID
IP Address 192.168.1. 0
IP Address (Binary) 11000000.10101000.00000001. 00000000
Subnet Mask 255.255.255. 0
Subnet Mask (Binary) 11111111.11111111.11111111. 00000000
Table 145 Subnet 1
IP/SUBNET MASK NETWORK NUMBER
LAST OCTET BIT
VALUE
IP Address 192.168.1. 0
IP Address (Binary) 11000000.10101000.00000001. 00000000
Subnet Mask 255.255.255. 128
Subnet Mask (Binary) 11111111.11111111.11111111. 10000000