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IP Switching Tech Tip
Assigning IP Addresses in IP Switching
Introduction
When planning the installation of an IP Swithcing network, you need to work
out the assignment of addresses for the IP Switch Controller, and the associated
IP Switch Gateways. Your goal is to take a small chunk of contiguous addresses
from your site's IP address space, and carve this chunk up into tiny point to
point subnets that connect each Gateway to the Controller in a private subnet.
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
- 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.
- 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.
- 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
In the above example, we had three Gateways, and got a block of 12 contiguous addresses.
This is the ideal arrangement, but is not absolutely necessary. The only requirement for
the assignment of addresses is in step one of the procedure. The least significant digit
of the base address of the block of four addresses must be divisible by four.
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