So I have a comcast cable modem.........

[bly]

Dynimically assigned IPs are "leased" from a DHCP server, the DHCP server in this case would be at the cable company.

If the lease expires and isn't renewed you can't get out on the IP.

I don't think this is your problem, but it's easy to test for.

go to start ~> Run and type "cmd" then ok
That will bring up a dos box

at the prompt type
ipconfig /all

this will show you your IP and other information, and when your lease was obtained and when it expires.

You'll see something like this:
Lease Obtained. . . . . . . . . . : Thursday, July 28, 2005 2:42:27 AM
Lease Expires . . . . . . . . . . : Sunday, July 31, 2005 2:42:27 AM

Leases can last a couple hours to a couple months, that all depends on how the DHCP server was setup, it's a configurable item.

A lease automatically tries to renew it's self once half the lease time has been used. So if the lease is good for 24 hours, at 12 hours it will try to renew it. You lease shouldn't ever run out, which is why I doubt this is an issue.

Ipconfig /release
ipconfig /renew
This will release the current DHCP lease and get a new one, if for some reason your DHCP isn't auto renewing (very rare), this will force it.

Next time you lose connection you can do an IPCONFIG /all to make sure your lease is still valid.

Again, I highly doubt your DHCP is the issue, I'd call comcast and have them run a level and packet loss test.