I think that your question and its answer would be a fine addition to our FAQs. Why don't you confer with Kilroy
to agree on its content and wording and one of you could submit it to a FAQ and get some extra brownie points here at DSLR for your submission. Don't think we can give points to both of you, but at least the other could get honorable mention in the FAQ itself?
Another reason or two in considering a router for even a single system:
It provides a backup for your software firewall if there is any kind of failure there (doesn't load properly OR with viruses and trojans targeting specific anti-virus and possibly software firewall applications), then if your software firewall is compromised at least there will be some protection from other malware just walking right end.
NOTE: if you were to become compromised in such a fashion, the one that took over your system could invite all its buddies in past the router after that, but at least you'd minimize damage to that caused by the one invader. Most likely way to get such an invader: by inviting it onto the system via opening bad email attachment, visiting a malware distributing site, downloading infected free-stuff on the internet.
It is "always on" when you have a connection - no waiting for any length of time at all for the software firewall to be loaded and active during a system boot.
Less chance of something running around on the internet looking for open systems being prepared to battle through both the router and the software firewall.
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I personally like these two Netgear units:
FVS-318 8-port Firewalling Router It's what I'm using right now.
Netgear FVS-114 The 4-port version, and
Netgear FRP-114P 4-port plus port to set up a printer as a networked device.
Other brands such as Linksys and Xyxel have similar offerings. I mention Netgear because it's what I have, I can verify that the FVS-318 does what it claims, and I'm happy with it.
Oh, and if you want to include any of my earlier "reasons" for choosing a Router for any home network, feel free to do so without even mentioning my name. Not a problem for me.