The Internet Protocol Journal - Volume 9, Number 2


A few of our eagle-eyed readers have pointed us to some errors in IPJ, Volume 9, Number 1. The text below Figure 6 on page 29 and continuing at the top of page 30 should read as follows:

The figure shows four hosts with the addresses,,, and Router A advertises, meaning: “Any host within the address range through is reachable through me.” Note that not all the hosts within this range exist, and that is okay—if a host within that range of addresses is reachable, it is reachable through Router A. In IP, the address that A is advertising is called a network address, and you can conveniently think of it as an address for the wire to which the hosts and router are attached, rather than a specific device.

For many people, the confusing part comes next. Router B is advertising, which is another network address. Router C can combine—or aggregate—these two advertisements into a single advertisement. Although we have just removed the correspondence between the wire and the network address, we have not changed the fundamental meaning of the advertisement itself. In other words, Router C is saying: “Any host within the range of addresses from through is reachable through me.” There is no wire with this address space, but devices beyond Router C do not know this, so it does not matter.

Also, Figure 8 on page 32 is reproduced here in its corrected form:

Figure 8: Subnet Chart