Secondary IP Addressing

Secondary IP Addressing

secondary IP addressing. Secondary addressing uses multiple networks or subnets on the same data link. By using more than one subnet in the same Layer 2 broadcast domain, you increase the number of available IP addresses.

shows the ideas behind secondary addressing. Hosts A and B sit on the same LAN, in fact, in the same VLAN. So does R1. No trunking needs to occur, either. In fact, if you ignore the numbers, normally, A, B, and R1 would all be part of the same subnet.

TCP/IP Network with Secondary Addresses

Secondary addressing allows some hosts to have addresses in one IP subnet, others to have addresses

in a second IP subnet, and the router to have addresses in both. Both IP subnets would be in the same Layer 2 broadcast domain (VLAN). As a result, the router will have connected routes for both the subnets, so the router can route packets to both subnets and even between both subnets.

Note that the second ip address command must have the secondary keyword, implementing secondary addressing, which tells router to add this as an additional IP address. Without this keyword, the router would replace the other IP address.

Secondary addressing does have one negative: Traffic between hosts on the same VLAN, but in different subnets, requires a trip through the router. For example, in Figure 16-14, when host A in subnet 172.16.1.0 sends a packet to host B, in subnet 172.16.9.0, host A’s logic is to send the packet to its default gateway. So, the sending host sends the packet to the router, which then sends the packet to host B, which is in the other IP subnet but in the same Layer 2 VLAN.