Hello all! D

I'm a bit crossed on how routers handle broadcast traffic. As I am aware, routers will pass broadcast messages within a LAN but not the WAN. Is this correct? Or do routers not pass broadcasts at all, not even on their own subnet? Thanks for taking the time to help!

Hi there,

very simple: They don't. ;-)

To understand this, you should be aware that the boxes sold as "routers" on the consumer market are actually compound devices - a combination of a router, a switch, and very often additional components (like a WLAN access point, a VoIP gateway). A switch links several networking devices in one network, a router links two distinct networks together.
Let's consider a unit with four LAN ports, one WAN, and a WLAN access point. A block diagram of it would look like this:

Broadcast messages are distributed normally within the LAN (which often includes the WLAN) by the Switch component, as every regular switch would do. Broadcast messages are also forwarded to the Router component. However, the router ignores them by design; it's not a router's job to route broadcasts.
Picking up the above remark in parantheses: There are so-called routers that set up a separate network for WLAN, which requires an extra Router block between the switch and the AP. The "Fritz" boxes used to do that with some older firmware releases; recently, however, that feature was dismissed. :-(

Does this clarify your confusion?

[X] Doc CPU

"Does this clarify your confusion?"

It sure does, and thanks for going all out on the explanation! Much appreciated, buddy! If I might expand the idea a bit more (yes, you are now a victim of your own awesomeness), if I were to turn off DHCP on my router (Linksys EA4500) I could serve DHCP from a CentOS6 server (HP ProLiant ml310) to other devices on my network through the router? How about if the server only has one NIC? Would that also cause a problem?

Thanks again for the response!

Hi there,

that's exactly what I'm doing in my home office setup: The router/modem connecting me to the internet via DSL would also supply DHCP, but I chose to turn that function off and instead I'm running a small Linux-based server (currently Gentoo, but I'm on the verge of installing it anew using Debian as a basis). This little box has DHCP, a local DNS, a local mail server, Apache, Samba - the full story. The advantage is that using a Linux-based PC as a DHCP server (I'm running dnsmasq for that purpose), I have much more control of the DHCP process than using the router's built-in DHCP server.

None that I can imagine at the moment. A router should have two NICs - one for each network it connects. However, from a theoretical point of view, a router (in the strict sense of the word) with only one NIC is possible as well. The practical fitness of such a router would be limited, of course, because it's the equivalent of a post office with only one counter where incoming and outgoing mail gets mixed.
But a server that only offers local services? What would be the point of having two NICs?

[X] Doc CPU

Thanks again for your reply, much appreciated. The project I was working on went nice and smooth thanks to your help!