If this is ICMP, I suspect it's normal.
I don't use a modem, but have a router. I do see ICMP pings from the router sometimes, usually at startup or resume from standby. More frequently I see ARP (address resolution protocol) in action.
Every few minutes the router sends a query to all devices "who has x.x.x.x IP?", and the box with that IP address replies "Me, here is my MAC address xx-xx-xx-xx-xx-xx", and the computers do the same about other computers, everybody keeps asking everybody else who is where.
If you don't see the outgoing, perhaps you're not logging it in the firewall. Or the firewall is setup to ignore the external pings, which is fine as well, so long as your connection is working.
What do the Norton logs look like besides this one entry? See, I don't have Norton (nor Vista), so a wording such as "port blocking allowed" is not very clear to me. Let's say you go to google, how do the logs read?
Edited by tos226, 09 May 2010 - 05:03 PM.