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How are routers on the network found?

Published: February 24, 2011

Applies To: Windows Small Business Server 2011 Standard

If the DHCP Server service is not enabled on the router, Windows SBS 2011 Standard tries to find a router as follows:

  • Windows SBS 2011 Standard sends an Internal Control Message Protocol (ICMP) router discovery message on the multicast address 224.0.0.2. Routers on the local network that support RFC 1256 immediately respond with a router advertisement.

    When a server or a client computer on the local network that supports RFC 1256 needs to locate a default gateway (router), the server or client computer uses ICMP to send a router solicitation. The router solicitation can be sent to the all routers IP multicast address, 224.0.0.2, which is the local IP broadcast address that IPv4 reserved. IPv4 multicast addresses in the range 224.0.0.0/24 (from 224.0.0.0 to 224.0.0.255) are reserved for the local subnet.

  • Windows SBS 2011 Standard uses the Simple Service Discovery Protocol (SSDP) to find routers that do not support RFC 1256.

    UPnP certified devices advertise their presence on the network to other devices and control points by using SSDP. The information that is exchanged between the device and the control point is limited to discovery messages that provide basic information about the devices and their services, and a description URL, which can be used to gather additional information about the device.

  • Windows SBS 2011 Standard sends Address Resolution Protocol (ARP) requests to 192.168.*.1 and 192.168.*.254. This checks 255 subnets for a router.

If a device is found, Windows SBS 2011 Standard attempts to determine if the device is a router as follows:

  • Pings the device to see if it responds.

  • Attempts to connect to the Internet through the device by making a DNS request to the root DNS server.

If the DHCP Server service on the router is turned on, Windows SBS 2011 Standard does the following:

  • Uses the DHCP server subnet settings on the router to set up the DHCP Server service on Windows SBS 2011 Standard. The DHCP Server service is configured to hand out Class C addresses on the same network as the router, from .1 to .254, excluding .1 to .10 (which are reserved for routers, servers, and other network devices).

  • Requests that the DHCP Server service be turned off manually on the router.

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