mistake in the real world. Other ranges are also reserved and should not be used. A now-common example is the use of the 169.254.0.0/16 network when DHCP requests go unanswered. RFC 3330 (Special-Use IPv4 Addresses) describes these networks. The entry for 169.254.0.0 is as follows: 169.254.0.0/16 - This is the "link local" block. It is allocated for communication between hosts on a single link. Hosts obtain these addresses by auto-configuration, such as when a DHCP server may not be found. Over the years, I have encountered more than one company whose entire scheme was based on the network 128.0.0.0 because it was easy to decipher in binary 10000000.00000000.00000000.00000000). This is a dangerous practice. Again, RFC 3330 provides the details: 128.0.0.0/16 - This block, corresponding to the numerically lowest of the former Class B addresses, was initially and is still reserved by the IANA. Given the present classless nature of the IP address space, the basis for the reservation no longer applies and ad- dresses in this block are subject to future allocation to a Regional Internet Registry for assignment in the normal manner. Similarly, you should never use the 127.0.0.0/8 network when designing IP networks. Though many people are familiar with the fact that 127.0.0.1 is a local loopback ad- dress, many are unaware that the entire network is reserved. RFC 3330 explains: 127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback, but no addresses within this block should ever appear on any network any- where [RFC1700, page 5]. Many companies design their IP networks with a complete disregard for the rules, often using IP ranges that correspond to addresses, building numbers, or branch numbers. The risks of such designs have to do with connectivity and the rules of routing. If a host on the improperly number