ISP Address Allocation

[rfuller () 3x com]

What is the best and most accurate way to determine an ISP's range of
addresses?  For example, if I wanted to see what AT&T's allocation of IP
addresses are, how would I go about doing this in the most timely manner?
What about for smaller ISPs?

Thanks!
Ron Fuller, CCDP, CCNP-ATM, CCNP-Security, MCNE, MCP
3X Corporation
rfuller () 3x com