Q & A on "Net 0" IP Addresses.

Q: Is it Ok to assign 0.xx.xx.xx to an IP host number, where xx != 0, i.e., class A with a prefix = all-0s and suffix is any thing, but not all-0's.

A: The question doesn't need an asnwer: addresses on the Internet are assigned by a central authority, so the question is irrelevant. Addresses on private networks are assigned to avoid global addresses (e.g., the IETF recommends using the class A address 10.0.0.0, which will never again be assigned to any network in the Internet). Thus, one would never assign 0.x.x.x in either case.

Q: But according to Tanenbaum, pp. 417, he said that when a prefix is all-0s and suffix is HOST, it means a host on this network. He added, these addresses allow machines to refer to their own network without knowing its number, but they have to know its class to know how many 0s to include.

A: That doesn't make sense. One cannot know the ``class'' of the local network address without knowing the address.

Q: This means, according to Tanenbaum, that 0.xx.xx.xx is a special address. However, I did not find it listed as a special address in Figure 16.7. That is why I got confused about this address.

A: There's no point in worrying about it because it's not used that way in practice.

Q: With reference to:

Is it Ok to assign 0.xx.xx.xx to an IP host number, where xx != 0, i.e., class A with a prefix = all-0s and suffix is any thing, but not all-0's.

Would it be a simple case of xx != 0? (Even with an ordinary class A prefix: 40.0.0.8) Obviously not all zeros, but some zeros are permitted.

A: I understood the question to be whether it was permissible to use the Class A prefix that consists of eight zero bits. At least some of the host part (the last three octets) will be nonzero, but that doesn't answer the question of whether Class A prefixes start at 0 or 1.