IP Address Allocation
5-4
8000-A2-GB21-20
November 1997
The following illustration is an example of structured subnet addressing.
97-15466-01
DSL Card
DSL Port 1
DSL Port 2
DSL Port 3
DSL Port 4
LAN Port
200.200.200.
n /
255.255.255.0
200.200.200.240 /
255.255.255.240
200.200.200.224 /
255.255.255.240
200.200.200.208 /
255.255.255.240
200.200.200.192 /
255.255.255.240
RTU1
RTU2
RTU3
RTU4
n = Any valid IP address, but not within the other subnets
To understand why this subnetting scheme works, you may want to consider the
IP addresses and subnet masks in hexadecimal:
Dotted Decimal
Dotted Hexadecimal
200.200.200.00 / 255.255.255.0
C8.C8.C8.00 / FF.FF.FF.00
200.200.200.240 / 255.255.255.240
C8.C8.C8.F0 / FF.FF.FF.F0
200.200.200.224 / 255.255.255.240
C8.C8.C8.E0 / FF.FF.FF.F0
200.200.200.208 / 255.255.255.240
C8.C8.C8.D0 / FF.FF.FF.F0
200.200.200.192 / 255.255.255.240
C8.C8.C8.C0 / FF.FF.FF.F0
In the previous illustration:
Each of the four DSL ports is on a different subnetwork of size 16, and the
subnet mask for the four ports is 255.255.255.240.
The LAN port (10BaseT port) IP address is 200.200.200.
n (where n can be
any valid IP address, but cannot be an IP address within the other subnets),
and its subnet mask is 255.255.255.0.
The illustration on page 5-5 shows one NSP connected to one DSL card, while
the illustration on page 5-6 shows 16 NSPs connected to one DSL card. On page
5-6, the NSP router is multihomed to support all 16 NSPs. Also, each RTU has 32
end-user systems (ES).
In summary, if 32 end-user systems are connected to the DSL card’s port 1 (s1c)
and all are using host addressing, then 32 host routes must be configured on the
RTU. If they are using structured subnet addressing, then only one route is
configured on the 5446 RTU. Remember that structured subnet addressing
applies only to the 5446 RTU.
