IP Address Design Example 1

Oct 12,2010 by admin

IP Address Design Example 1


	Assume your company has been assigned the Class C address 198.28.61.0 and you have determined that you require four networks with a maximum of 25 hosts per network. From Table 2-6, you will need three subnet bits, resulting in a subnet mask of 255.255.255.224. The subnet numbers for this design are any four of the following, as shown in Figure 2-4.


	Figure 2-4: IP address design example 1


	198.28.61.32

198.28.61.64

198.28.61.96

198.28.61.128

198.28.61.160

198.28.61.192


	Although subnets solve some of the problems associated with the inefficient use of IP address space, situations occur when simple subnetting does not suffice. Consider the network in Figure 2-5 in which two routers are connected by a serial link. This serial link is a point-to-point connection, so there are only two hosts on the link, the two router interfaces. Each network must also be on a separate subnet, so no matter which subnet mask we choose, we will be wasting IP addresses. If we are using a Class B address with a 24-bit subnet mask, then the subnet assigned to the serial link will only use two out of a possible 254 host addresses.


	Figure 2-5: Limitations of simple subnetting

If we could use different subnet masks for different subnetworks, then the limitations of Figure 2-5 could be solved. A subnet mask of 255.255.255.252 (or /30) can accommodate only two hosts, which is perfect for a point-to-point serial link. Unfortunately, this mask, if used throughout the network, would limit all subnets to two hosts. The ideal solution would be to vary the length of the subnet mask and adjust it according to the needs of each individual network.