ZyXEL Communications 128MH user manual Note on IP Address Assignment, IP Subnet Mask

Prestige 128MH PSTN Router/Hub

Note on IP Address Assignment

A unique 32-bit IP address is assigned to each host on the Internet. Similarly, every machine on an internet must have a unique IP address. Do not assign an arbitrary address to any machine on your network without prior consulting your network administrator.

IP Subnet Mask

A subnet mask is a 32-bit quantity that, when logically ANDed with an IP address, yields the network number. For instance, the subnet masks for class A, B and C without subnetting are 255.0.0.0, 255.255.0.0 and 255.255.255.0, respectively.

The subnet mask is used to split the IP network addresses to create more network numbers. More network numbers can be created by shifting some bits from the host ID to the network ID. For instance, to partition a class C network number 192.68.135.0 into two, you shift 1 bit from the host ID to the network ID. Thus the new subnet mask will be 255.255.255.128; the first subnet will have network number 192.68.135.0 with hosts 192.68.135.1 to 192.68.135.126 and the second subnet will have network number 192.68.135.128 with hosts 192.68.135.129 to 192.68.135.254.

It is recommended that you use the same subnet mask for all physical networks that share an IP network number. Table 3-1 below lists the additional subnet mask bits in dot decimal notations. To use Table 3-1, write down the original subnet mask and substitute the higher order 0s with the dot decimal of the additional subnet bits. For instance, to partition your class C network 204.247.203.0 with subnet mask 255.255.255.0 into 16 subnets (4 bits), the new subnet mask becomes 255.255.255.240.

Table 3-1. Subnet Mask Notation

Additional Subnet Mask Bits in Dot Decimal Notation

Number of Bits

Dot Decimal Value

1

128

2

192

3-2Chapter 3 Internet Access Application