ork design is not something that many people will appreciate at all, until it is explained to them. Still, careful planning and design are required to create an IP schema that will allow growth for years to come. Public Versus Private IP Space If you've taken any certification exams, you should be familiar with the idea of pub- lic versus private IP space. In a nutshell, there exist a select number of IP networks that should not be routed on the public Internet. These networks are described in RFC 1918 ("Address Allocation for Private Internets") as follows: 3. Private Address Space The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets: 10.0.0.0 10.255.255.255 (10/8 prefix) 172.16.0.0 172.31.255.255 (172.16/12 prefix) 192.168.0.0 192.168.255.255 (192.168/1 prefix) A quick note about these ranges is in order. First, the 172.16.0.0 network is not just the /16 range, and it is not the 172.0.0.0/8 range. The range allocated for 172.16.0.0 is a /12. The IP range included in this private range is 172.16.0.0-172.31.255.255 The ranges 172.0.0.0-172.31.255.255 and 172.32.0.0-172.255.255.255 are not com- posed of private addresses, and should not be used as such. This is a mistake commonly made 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 - T