5. Example of new allocation and routing
5.1 Address allocation
Consider the block of 2048 class C network numbers beginning with
192.24.0.0 (0xC0180000 and ending with 192.31.255.0 (0xC01FFF00)
allocated to a single network provider, "RA". A "supernetted" route
to this block of network numbers would be described as 192.24.0.0
with mask of 255.248.0.0 (0xFFF80000).
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RFC 1519 CIDR Address Strategy September 1993
Assume this service provider connects six clients in the following
order (significant because it demonstrates how temporary "holes" may
form in the service provider's address space):
"C1" requiring fewer than 2048 addresses (8 class C networks)
"C2" requiring fewer than 4096 addresses (16 class C networks)
"C3" requiring fewer than 1024 addresses (4 class C networks)
"C4" requiring fewer than 1024 addresses (4 class C networks)
"C5" requiring fewer than 512 addresses (2 class C networks)
"C6" requiring fewer than 512 addresses (2 class C networks)
In all cases, the number of IP addresses "required" by each client is
assumed to allow for significant growth. The service provider
allocates its address space as follows:
C1: allocate 192.24.0 through 192.24.7. This block of networks is
described by the "supernet" route 192.24.0.0 and mask
255.255.248.0
C2: allocate 192.24.16 through 192.24.31. This block is described
by the route 192.24.16.0, mask 255.255.240.0
C3: allocate 192.24.8 through 192.24.11. This block is described
by the route 192.24.8.0, mask 255.255.252.0
C4: allocate 192.24.12 through 192.24.15. This block is described
by the route 192.24.12.0, mask 255.255.252.0
C5: allocate 192.24.32 and 192.24.33. This block is described by
the route 192.24.32.0, mask 255.255.254.0
C6: allocate 192.24.34 and 192.24.35. This block is described by
the route 192.24.34.0, mask 255.255.254.0
Note that if the network provider uses an IGP which can support
classless networks, he can (but doesn't have to) perform
"supernetting" at the point where he connects to his clients and
therefore only maintain six distinct routes for the 36 class C
network numbers. If not, explicit routes to all 36 class C networks
will have to be carried by the IGP.
To make this example more realistic, assume that C4 and C5 are
multi-homed through some other service provider, "RB". Further assume
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RFC 1519 CIDR Address Strategy September 1993
the existence of a client "C7" which was originally connected to "RB"
but has moved to "RA". For this reason, it has a block of network
numbers which are allocated out "RB"'s block of (the next) 2048 class
C network numbers:
C7: allocate 192.32.0 through 192.32.15. This block is described
by the route 192.32.0, mask 255.255.240.0
For the multi-homed clients, we will assume that C4 is advertised as
primary via "RA" and secondary via "RB"; C5 is primary via "RB" and
secondary via "RA". To connect this mess together, we will assume
that "RA" and "RB" are connected via some common "backbone" provider
"BB".
Graphically, this simple topology looks something like this:
C1
192.24.0.0 -- 192.24.7.0 \ _ 192.32.0.0 - 192.32.15.0
192.24.0.0/255.255.248.0 \ / 192.32.0.0/255.255.240.0
\ / C7
C2 +----+ +----+
192.24.16.0 - 192.24.31.0 \| | | |
192.24.16.0/255.255.240.0 | | _ 192.24.12.0 - 192.24.15.0 _ | |
| | / 192.24.12.0/255.255.252.0 \ | |
C3 -| |/ C4 \| |
192.24.8.0 - 192.24.11.0 | RA | | RB |
192.24.8.0/255.255.252.0 | |___ 192.24.32.0 - 192.24.33.0 ___| |
/| | 192.24.32.0/255.255.254.0 | |
C6 | | C5 | |
192.24.34.0 - 192.24.35.0 | | | |
192.24.34.0/255.255.254.0 | | | |
+----+ +----+
\\ \\
192.24.12.0/255.255.252.0 (C4) || 192.24.12.0/255.255.252.0 (C4) ||
192.32.0.0/255.255.240.0 (C7) || 192.24.32.0/255.255.254.0 (C5) ||
192.24.0.0/255.248.0.0 (RA) || 192.32.0.0/255.248.0.0 (RB) ||
|| ||
VV VV
+--------------- BACKBONE PEER BB ---------------+