Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 78
Example Two: Relaxing the Assignment

The previous assignment works, but is it really good? Although we were able to be very stringent with the address assignment, this is not a good way of assigning or masking the address. It does not leave much room for growth on the host side or on the network side. For example, what if the company expands to 100 hosts per subnet and requires two more subnets? It could call its ISP back and request another address assignment. But by now, the ISP has handed out a few more addresses and the next available address for the customer is 200.24.64.0/24. This is not contiguous with the original assignment and the ISP has to add another entry in the ISP’s table when this could have been avoided. To anticipate for this expansion, the customer could have been assigned four Class Cs. The ISP block assigned to the customer could be 200.1.252.0/22 (one entry in the ISP routing table), which yields the Class C addresses of 200.1.252.0, 200.1.253.0, 200.1.254.0, and 200.1.255.0. The customer is free to assign any subnet mask he or she wishes to the addresses without notifying the ISP.

From here the customer could assign 1 bit of subnet mask on the address of 200.1.252.0, which allows for 7 bits of address space yielding 125 (2ⁿ7 –2) hosts per subnet. The other address could remain intact or be split with 1 bit subnet mask. The customer could also have simply used all the bits in the fourth octet, using no subnet mask. Yes, 1 bit subnet mask is allowed on a Class C: a 0 subnet and a 1 subnet. Review RFC 1812. The only time this will lead to problems is if the site is using all subnets broadcast. However, check with your router vendor. Cisco does not support 1-bit subnet masks. In this case, you will have to use the Class C assignment with subnets. With VLSM, the consumer would have to devise a plan to determine which subnets will only have 60 hosts and which require more.

Example Two: Relaxing the Assignment

This is a simple example of how you must think about your network design before calling an ISP. You need to know how many hosts and what the traffic patterns are on the network. IP addresses are in short supply and ISPs do not hand them out haphazardly. They must take into consideration their routing tables as well.