IP Switching Tech Tip

Assigning IP Addresses in IP Switching

Introduction

The diagram above shows a basic IP Switching installation consisting of three IP Switch Gateways and one IP Switch Controller. The Gateways in this example are routing traffic from three Fast Ethernet subnets, but that is not the focus of this note. We're interested in the addresses assigned to the ATM interfaces that connect the Gateways and Controllers to the GIGAswitch/ATM.

Procedure

1. Determine the number of Gateways in your installation. For each Gateway you will need FOUR contiguous IP Addresses. The first address in the list of four must be evenly divisible by four.

   Basically, you're only concerned with the last digit of your IP addresses in this case. IP addresses are of the form: blah.blah.blah.NUMBER you care about the last number. In the example above, the TOP gateway got assigned the addresses blah.blah.blah.4, blah.blah.blah.5, blah.blah.blah.6, and blah.blah.blah.7 To keep this simpler, we'll say it got addresses 4,5,6, and 7.

   The key is that the base address 4 is evenly divisible by 4.

2. Make a table that looks like the following:

   IP Address	Binary Equivalent	Role
   *.*.*.4	00000100	Network Number
   *.*.*.5	00000101	Controller
   *.*.*.6	00000110	Gateway 1
   *.*.*.7	00000111	Broadcast Address

   If you examine the diagram above, you'll note that the Controller was assigned address 5 and the Gateway got address 6. This assignment of addresses and their roles is the consequence of using a 30-bit network mask, as instructed in the GIGAswitch/IP Installation and Configuration manual. A 30-bit IP address mask leaves just TWO bits to identify the nodes in a subnet. The binary equivalents shown in the table demonstrate the effect of the netmask. The 30-bit netmask leaves only the last two binary digits (shown in bold type in the table) for the node number. So, node 00 is unused, node 01 becomes the Controller node, node 10 becomes the Gateway node, and 11 is the Broadcast address for this tiny subnet. Since the subnet is point to point, the broadcast address is unused.

3. Extend your table to cover all of your Gateways:

   IP Address	Binary Equivalent	Role
   *.*.*.4	00000100	Network Number
   *.*.*.5	00000101	Controller
   *.*.*.6	00000110	Gateway 1
   *.*.*.7	00000111	Broadcast Address
   *.*.*.8	00001000	Network Number
   *.*.*.9	00001001	Controller
   *.*.*.10	00001010	Gateway 2
   *.*.*.11	00001011	Broadcast Address
   *.*.*.12	00001100	Network Number
   *.*.*.13	00001101	Controller
   *.*.*.14	00001110	Gateway 3
   *.*.*.15	00001111	Broadcast Address

Notes