IP Address Allocation
5-4
8000-A2-GB21-30
April 1998
Structured Subnet Addressing
As an alternative to using host routes for end-user systems, structured subnetting
can be used. It scales better and performs better, but it does not allow
geographically dispersed subnets.
NOTE:
Structured subnetting is supported on the 8546 DSL card and the 5446 RTU.
It is not supported, however, on the 8540 DSL card and its corresponding
RTUs on the DSL ports. For 5546 RTUs, the router attached to the RTU must
be configured with structured subnets.
The Network Service Provider’s IP address itself is a host IP address which uses
a subnet mask. This subnet mask includes all interfaces from the 5446 RTU.
Structured subnet addressing uses the following method:
H Within the service domain, the NSP would provision a subnet of its domain to
a DSL card and all devices behind it.
H The NSP would further subdivide that subnet into four additional subnets
(one behind each DSL port).
The following illustration is an example of structured subnet addressing.
97-15466-0
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
