Windows NT Routing Basics

This article discusses the Windows NT Routing Table on a single-homed machine and multihomed Windows NT Router with and without Multi Protocol Router (MPR). This background information will help with troubleshooting related to TCP/IP.


MORE INFORMATION

The Route Table

Even a single-homed TCP/IP host has to make routing decisions. These routing decisions are controlled by the route table. The route table can be displayed by typing "route print" at the command prompt. The following is an example route table from a single-homed machine. This simple route table is built automatically by Windows NT based on the IP configuration of your host. 

Network Address   Netmask          Gateway Address  Interface       Metric

0.0.0.0           0.0.0.0          157.57.8.1       157.57.11.169      1
127.0.0.0         255.0.0.0        127.0.0.1        127.0.0.1          1
157.57.8.0        255.255.248.0    157.57.11.169    157.57.11.169      1
157.57.11.169     255.255.255.255  127.0.0.1        127.0.0.1          1
157.57.255.255    255.255.255.255  157.57.11.169    157.57.11.169      1
224.0.0.0         224.0.0.0        157.57.11.169    157.57.11.169      1
255.255.255.255   255.255.255.255  157.57.11.169    157.57.11.169      1


The titles of each column in the above table are explained in following text:

Network Address:

Network Address is the destination. The Network address column can contain:

The search order is also as above, from the most unique route (host address) to most generic (default gateway): 

0.0.0.0         is the default route
127.0.0.0       is the loopback address
157.57.8.0      is the local subnet address
157.57.11.169   is the network card address
157.57.255.255  is the subnet broadcast address
224.0.0.0       is the multicast address
255.255.255.255 is the limited broadcast address


Netmask:

The Netmask defines what portion of the Network Address must match for that route to be used. When the mask is written in binary a 1 is significant (must match) and a 0 need not match. For example, a 255.255.255.255 mask is used for a host entry. The mask of all 255s (all 1s) means that the destination address of the packet to be routed must exactly match the Network Address for this route to be used. For another example, the Network Address 157.57.8.0 has a netmask of 255.255.248.0. This netmask means the first two octets must match exactly, the first 5 bits of the third octet must match (248=11111000) and the last octet does not matter. Since 8 in the decimal number system is equivalent to 00001000 in binary, a match would have to start with 00001. Thus, any address of 157.57 and the third octet of 8 through 15 (15=00001111) will use this route. This is a netmask for a subnet route and is therefore called the subnet mask.

Gateway Address:

The Gateway Address is where the packet needs to be sent. This can be the local network card or a gateway (router) on the local subnet.

Interface:

The Interface is the address of the network card over which the packet should be sent out. 127.0.0.1 is the software loopback address.

Metric:

The Metric is the number of hops to the destination. Anything on the local LAN is one hop and each router crossed after that is an additional hop. The Metric is used to determine the best route.

Multihomed Router

The following is the default Route table of a multihomed Windows NT host. 

Network Address   Netmask           Gateway Address   Interface      Metric

0.0.0.0           0.0.0.0           157.57.24.1       157.57.24.193     1
0.0.0.0           0.0.0.0           199.199.40.1      199.199.40.139    1
127.0.0.0         255.0.0.0         127.0.0.1         127.0.0.1         1
157.57.24.0       255.255.248.0     157.57.24.193     157.57.24.193     1
157.57.24.193     255.255.255.255   127.0.0.1         127.0.0.1         1
199.199.40.0      255.255.255.0     199.199.40.139    199.199.40.139    1
199.199.40.139    255.255.255.255   127.0.0.1         127.0.0.1         1
199.199.40.255    255.255.255.255   199.199.40.139    199.199.40.139    1
224.0.0.0         224.0.0.0         157.57.24.193     157.57.24.193     1
224.0.0.0         224.0.0.0         199.199.40.139    199.199.40.139    1
255.255.255.255   255.255.255.255   199.199.40.139    199.199.40.139    1


Check "Enable IP Routing" in the Advanced TCP/IP configuration to enable routing. At this point Windows NT will route between these two subnets.

NOTE - for the Multihomed Router to pass DHCP Discover packets from one subnet to the other you will need to install the "BootP Relay Agent" that comes with MPR. MPR is discussed later.

A note on Default gateways:

In the TCP/IP configuration, you can add a default route for each network card. This will create a 0.0.0.0 route for each. However, only one default route will actually be used. In this case, the 199.199.40.139 is the first card in the TCP/IP bindings and therefore the default route for this card will be used. Since only one default gateway will be used you should only configure one card to have a default gateway. This will reduce confusion and insure the results you intended.

For more information, see the following article in the Microsoft Knowledge base:
ARTICLE-ID: Q157025
TITLE : Default Gateway Configuration for Multi-Homed Computers

If the Windows NT router does not have an interface on a given subnet, it will need a route to get there. This can be done by adding Static Routes or by using MPR. MPR is discussed later.

To Add a Static Route

The following is an example route.

Route Add 199.199.41.0 mask 255.255.255.0 199.199.40.1 metric 2

NOTE: The metric option is only supported in Windows NT 3.51 with Service Pack 2 or later installed.

The route in this example means that to get to the 199.199.41.0 subnet with a mask of 255.255.255.0 use gateway 199.199.40.1 and that the subnet is 2 hops away. A static route will also need to be added on the next router telling it how to get back to subnets reachable by the first router. With a network of a few routers or more, static routes can become very complicated.

For additional information, please see the following article(s) in the Microsoft Knowledge Base:
ARTICLE-ID: Q141383
TITLE : "P" Switch for Route Command Added in Windows NT

Multi Protocol Router (MPR)

MPR consists of the following:
RIP is used by routers to dynamically exchange routing information. RIP routers broadcast their routing tables every 30 seconds by default. Other RIP routers will listen for these RIP broadcasts and update their own route tables.