As of January 22, 2002, comcast.com became comcast.net, immediately cutting down speeds by a factor of 3, creating several outages in the first month for me, and creating the a obscure way for non-dummies to setup their networking parameters. Luckily their e-tech support has been reasonably forthcoming, some of them even know how Linux is spelled :-) The network speed was finally back to 'normal' a few months later.

Here's a example from redhat's script that configures your networking. In my case it is in the file, /etc/sysconfig/network/network-scripts/ifcfg-eth0 you can edit it manually, but GUI's like netcfg (prior to rh7) or redhat-config-network in rh7+ will do this for you. Your details may differ. Also sure you don't have a GATEWAY set in /etc/sysconfig/network/network , since DHCP will set that for you: 

DEVICE=eth0			<-- eth0 or eth1 ..... whichever
BOOTPROTO=dhcp			<-- Comcast appears to use standard DHCP
BROADCAST=
IPADDR=
NETMASK=
NETWORK=
ONBOOT=yes			<-- pick yes or no
GATEWAY=			<-- make sure this is blank (check you ../network file also)
TYPE=Ethernet
FORWARD_IPV4=yes		<-- only if you have a local LAN
USERCTL=no			<-- that's up to you
PEERDNS=yes			<-- will set DNS for you
DHCP_HOSTNAME=cc12345-a		<-- you need to know your "CC" hostname
If you like totally manual setup, the following command would do that: (and this would be distribution independant) 
     dhcpd -h cc12345-a eth0
You probably also need to know a few mail and news server names: 
	mail:  mail.comcast.net  (POP3)  [some claim you may need e.g. mail.nrockv01.md.comcast.net]
	smtp:  smtp.comcast.net
	news:  news.md.comcast.giganews.com (i think)
Links:
comcast upgrade details
GigaNews, their outsourced usenet service, limited to 1GB/month, or 30MB/day, or 300 seconds/day download time!!!
IP: Comcast spy software?