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

Chapter 89
Special Subnet Considerations

RFC 950 (page 5) states that we should preserve the all 0s and all 1s in the subnet field, for they have special meaning in the certain fields indicated by IANA-assigned RFC numbers. For example, the address 130.1.255.255 could be interpreted as meaning all hosts on the network 130.1, or the address 0.0.0.1 could be interpreted as meaning host 1 on this network.

It is useful to preserve and extend the interpretation of these special addresses in subnetted networks. This means the values of all 0s and all 1s in the subnet field should not be assigned to actual (physical) subnets.

Due to increasing demand to make full use of all of the bits in the 32-bit wide address, subnet 0s and 1s are allowed. However, you must exercise caution when doing so. RFC 1812 (Requirements for IPv4 Routers) states:

All-subnets broadcasts (called multisubnet broadcasts) have been deprecated. . . . In a CIDR routing domain, wherein classical IP network numbers are meaningless, the concept of an all-subnets-directed-broadcast is also meaningless.

Basically, there are not subnets in CIDR.

Now, while the preceding extract was talking about the CIDR router domain, it could be misread by any routed domain. Many router vendors interpret RFCs different ways. For example, 3Com has the ability to turn ASB (All Subnets Broadcast) routing on or off, thereby allowing all 1s subnetwork number free to be assigned.

You may think, why would you want to place an ASB? This can come in handy when multicasting. As of this writing, the multicast protocols are not being used on customer networks, mainly due to inexperience and nervousness of the router support staff and its management. Routed networks are tricky enough without thoroughly understanding multicasting. Therefore, multicast application software vendors support ASB to route their information in a nonmulticast network. Unruly, yes, but it works.

This thinking may be propagated down to the lowest levels of routing in the Internet: the customer AS. If the customer AS has “deprecated” ASB, then you will be implementing all 0s and all 1s subnets. However, if a customer network has implemented it (all 1s subnets), then a packet addressed to an ASB will be routed to the subnet represented by the all 1s.