ISP Address Allocation

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