This article discusses the Windows NT Routing Table on a single-homedmachine and multihomed Windows NT Router with and without Multi ProtocolRouter (MPR). This background information will help with troubleshootingrelated to TCP/IP.
The Route Table
Even a single-homed TCP/IP host has to make routing decisions. Theserouting decisions are controlled by the route table. The route table canbe displayed by typing "route print" at the command prompt. The followingis an example route table from a single-homed machine. This simple routetable is built automatically by Windows NT based on the IP configurationof your host.
Network Address Netmask Gateway Address Interface Metric0.0.0.0 0.0.0.0 18.104.22.168 22.214.171.124 1127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1126.96.36.199 255.255.248.0 188.8.131.52 184.108.40.206 1220.127.116.11 255.255.255.255 127.0.0.1 127.0.0.1 118.104.22.168 255.255.255.255 22.214.171.124 126.96.36.199 1188.8.131.52 184.108.40.206 220.127.116.11 18.104.22.168 1255.255.255.255 255.255.255.255 22.214.171.124 126.96.36.199 1
The titles of each column in the above table are explained in followingtext:
Network Address is the destination. The Network address column cancontain:
- Host address
- Subnet address
- Network address
- Default gateway
The search order is also as above, from the most unique route (hostaddress) to most generic (default gateway):
0.0.0.0 is the default route127.0.0.0 is the loopback address188.8.131.52 is the local subnet address184.108.40.206 is the network card address220.127.116.11 is the subnet broadcast address18.104.22.168 is the multicast address255.255.255.255 is the limited broadcast address
The Netmask defines what portion of the Network Address must match forthat route to be used. When the mask is written in binary a 1 issignificant (must match) and a 0 need not match. For example, a255.255.255.255 mask is used for a host entry. The mask of all 255s (all1s) means that the destination address of the packet to be routed mustexactly match the Network Address for this route to be used. For anotherexample, the Network Address 22.214.171.124 has a netmask of 255.255.248.0.This netmask means the first two octets must match exactly, the first 5bits of the third octet must match (248=11111000) and the last octet doesnot matter. Since 8 in the decimal number system is equivalent to 00001000in binary, a match would have to start with 00001. Thus, any address of157.57 and the third octet of 8 through 15 (15=00001111) will use thisroute. This is a netmask for a subnet route and is therefore called thesubnet mask.
The Gateway Address is where the packet needs to be sent. This can be thelocal network card or a gateway (router) on the local subnet.
The Interface is the address of the network card over which the packetshould be sent out. 127.0.0.1 is the software loopback address.
The Metric is the number of hops to the destination. Anything on the localLAN is one hop and each router crossed after that is an additional hop.The Metric is used to determine the best route.
The following is the default Route table of a multihomed Windows NT host.
Network Address Netmask Gateway Address Interface Metric0.0.0.0 0.0.0.0 126.96.36.199 188.8.131.52 10.0.0.0 0.0.0.0 184.108.40.206 220.127.116.11 1127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 118.104.22.168 255.255.248.0 22.214.171.124 126.96.36.199 1188.8.131.52 255.255.255.255 127.0.0.1 127.0.0.1 1184.108.40.206 255.255.255.0 220.127.116.11 18.104.22.168 122.214.171.124 255.255.255.255 127.0.0.1 127.0.0.1 1126.96.36.199 255.255.255.255 188.8.131.52 184.108.40.206 1220.127.116.11 18.104.22.168 22.214.171.124 126.96.36.199 1188.8.131.52 184.108.40.206 220.127.116.11 18.104.22.168 1255.255.255.255 255.255.255.255 22.214.171.124 126.96.36.199 1
Check "Enable IP Routing" in the Advanced TCP/IP configuration to enablerouting. At this point Windows NT will route between these two subnets.
NOTE - for the Multihomed Router to pass DHCP Discover packets from onesubnet to the other you will need to install the "BootP Relay Agent" thatcomes with MPR. MPR is discussed later.
A note on Default gateways:
In the TCP/IP configuration, you can add a default route for each networkcard. This will create a 0.0.0.0 route for each. However, only one defaultroute will actually be used. In this case, the 188.8.131.52 is the firstcard in the TCP/IP bindings and therefore the default route for this cardwill be used. Since only one default gateway will be used you should onlyconfigure one card to have a default gateway. This will reduce confusionand insure the results you intended.
For more information, see the following article in the Microsoft Knowledgebase:
TITLE : Default Gateway Configuration for Multi-Homed Computers
If the Windows NT router does not have an interface on a given subnet, itwill need a route to get there. This can be done by adding Static Routesor by using MPR. MPR is discussed later.
To Add a Static Route
The following is an example route.
Route Add 184.108.40.206 mask 255.255.255.0 220.127.116.11 metric 2
NOTE: The metric option is only supported in Windows NT 3.51 with ServicePack 2 or later installed.
The route in this example means that to get to the 18.104.22.168 subnetwith a mask of 255.255.255.0 use gateway 22.214.171.124 and that the subnetis 2 hops away. A static route will also need to be added on the nextrouter telling it how to get back to subnets reachable by the firstrouter. With a network of a few routers or more, static routes can becomevery complicated.
For additional information, please see the following article(s) in theMicrosoft Knowledge Base:
TITLE : "P" Switch for Route Command Added in Windows NT
Multi Protocol Router (MPR)
MPR consists of the following:
- Routing Information Protocol (RIP) for TCP/IP.
- BOOTP (Boot Protocol) relay agent for Dynamic Host Configuration Protocol (DHCP).
- RIP for IPX.
RIP is used by routers to dynamically exchange routing information. RIProuters broadcast their routing tables every 30 seconds by default. OtherRIP routers will listen for these RIP broadcasts and update their ownroute tables.
MPR is available in Service Pack 2 for Windows NT 3.51.
For additional information, please see the RIPROUTE.WRI file (availablewith Windows NT 3.51 Service Pack 2) for MPR installation instructions orthe following article in the Microsoft Knowledge Base:
TITLE : Multi-Protocol Router Installation and Configuration