DOS Attack?

[Shawn Wall <sjwall () shaw ca>]

Hi List,

I'm currently experiencing network outages due to what appears to be DOS
attacks. I'm running a wireless ISP using a Cisco 2611 and CBAC and I have a
/24 public address range. During the outage I can see traffic from a single
external host sending thousands of packets to a single internal host. I
don't have port 80 inbound open in my ACLs so I don't understand how the
external host is even able to contact the internal host to begin with.
Secondly, how is it possible for an attack on 1 internal host to cripple the
rest of my network? Any feedback would be welcome. Thanks.

shawn