ISPs generally decline transfers of IP addresses especially when class C addresses
are involved.Transferring a block of IP address classes should be avoided as it
loads the IP routing tablesif fragmentation occurs. Whenever possible IP addresses
are keept in aggregates that need less space in the routing tables. For example,
16 class C blocks that have 4 common bits in the network part can be exported (by
the routing protocols) as a single entry as long as they 'belong' to the same ISP
as far as the rest of the world is concerned. All packets will be routed to the same
router(s).
If all address blocks were exported stand-alone then routers will have
to keep millions of entries (for class C addresses) in their routing tables. This
is unacceptable.
If these addresses were not exported aggregated with other addresses
then it would be no technical reason to not transfer IP addresses. However, that
will happen only to class A and B addresses.
BTW, assuming a fixed IP address is
a bad networking practice. That's why protocols like RARP/BOOTP/DHCP and DNS/WINS/LDAP
were developed, to allow hosts to pick up an IP address and to make human accessible
names invariant to IP address change.