size of the routing table is a big deal, especially in IPv6

Tue Nov 30 19:23:21 UTC 2004

Please see an earlier write-up below. Will we run into IPv6 routing table
problems without more formalized aggregation guidelines?

"The general guiding principal for the allocation of IPv6 address space is
as follows:

"     /48 in the general case, except for very large subscribers
"     /64 when it is known that one and only one subnet is needed by design
"     /128 when it is absolutely known that one and only one device is
connecting

The second-half of the IPv6 address (the last 64 bits) are generally
reserved for the devices end-system identifier. Typically this is a Network
Service Access Point (NSAP) identifier, or derived from the device's 48-bit
IEEE 802 address padded up to 64 bits. A /64 network address then can hold
64 bits worth of devices. It is useful to look at /64s as if they were IPv4
Class C addresses, which are typically used for a single LAN or broadcast
domain.

A /48 has 16 bits worth (65,536) of Class C's then, which, if we continue
the comparison, makes it the equivalent of an IPv4 Class A address. A /48
is generally considered the smallest routable prefix. A typical IPv6
address allocation (e.g., 200A:06D1::/32) holds 16 bits worth (65,535) of
these /48 Class A's. The overall Aggregatable Global Unicast range
(001::/3) holds 45 bits worth of routable networks. This clearly exceeds
any router's capacity to accommodate a routing table of strictly /48s."

Joe Loiacono