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Background: I just ordered a static IP address from my ISP. I now have to configure my router to use the static IP, but I'm not sure why.

Why does the router need to know the public IP address if it just connects directly to the modem?

My assumption was that a router just passes packets directly to the modem on layer 2, therefore it could communicate like a switch with MAC addresses. It wouldn't need to know the IP address of the modem. Is this logic incorrect?

If so, how exactly does the router communicate with the modem?

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2 Answers 2

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I've been doing some more research/thinking, and I believe I might have an answer:

The modem is configured in "bridge" mode and only communicates on layer 2--PPPoE to the ISP and Ethernet (MAC addressing) to the LAN. My terminology may not be perfect, but I do believe the modem is only functioning on layer 2 when configured in bridge mode, so it is not aware of IP addresses (layer 3).

So if the modem is simply passing along packets on layer 2, the router must make the layer 3 connection. Because I now have a static IP address, I need to configure the router to use that static IP address, instead of relying on DHCP to automatically obtain a dynamic IP address from the ISP.

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  • Your ISP may be able to assign you a static IP with DHCP, which is much more robust than using manual host configuration.
    – Nevin Williams
    Commented Oct 22, 2014 at 19:37
  • My thoughts exactly. In fact you want this. If you have a static IP you don't want to futz around with a double NAT (one from router, one from modem)
    – Rich Homolka
    Commented Oct 22, 2014 at 23:33
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Simplified, classically, it goes something like this:

  • Modems (modulator/demodulator pairs) are used to communicate over >1 modes of PHY, without participating in LNK.
    VT102 - modem - POTS - modem - VAX
    host - [ modems] - host
  • Bridges are used to communicate over >1 modes of LNK, without participating in NET.
    computer - ethernet - cable modem - DOCSIS - CMTS - ethernet - DHCP Server
    host - [etheret bridges] - host
  • Routers are used to communicate over >1 modes of NET, without participating in TRN.
    TV Settop Box - WiFi - SOHO gateway - FTTH - ISP gateway - CDN gateway - Netflix
    host - [routers] - host

It goes up and on through the layers, but there are few practical examples of simple discrete modems, bridges, and routers to begin with.

Dialup modems eventually began managing link functions such as compression and flow control... The simplest commercial cable modems of the mid-90s were always bridges,though now they often run nearly the full gamut, some even capable of content filtering. Fortunately, many multi-function devices can have their higher capabilities disabled, allowing those functions to be administrated elsewhere, which seems to be the case here, as you now administrate your own routing, including NAT and QoS, instead of your ISP doing so at your 'modem'.

The OSI model has never been quite discrete in practice, though emergent technologies are usually more delineated while standards and best common practices are getting hashed out.

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