TCP/IP Routing Basics for Windows NT

This article was previously published under Q140859
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SUMMARY
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.
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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          157.57.8.1       157.57.11.169      1127.0.0.0         255.0.0.0        127.0.0.1        127.0.0.1          1157.57.8.0        255.255.248.0    157.57.11.169    157.57.11.169      1157.57.11.169     255.255.255.255  127.0.0.1        127.0.0.1          1157.57.255.255    255.255.255.255  157.57.11.169    157.57.11.169      1224.0.0.0         224.0.0.0        157.57.11.169    157.57.11.169      1255.255.255.255   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 followingtext:

Network Address:

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 address157.57.8.0      is the local subnet address157.57.11.169   is the network card address157.57.255.255  is the subnet broadcast address224.0.0.0       is the multicast address255.255.255.255 is the limited broadcast address				


Netmask:

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 157.57.8.0 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.

Gateway Address:

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.

Interface:

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.

Metric:

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.

Multihomed Router

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           157.57.24.1       157.57.24.193     10.0.0.0           0.0.0.0           199.199.40.1      199.199.40.139    1127.0.0.0         255.0.0.0         127.0.0.1         127.0.0.1         1157.57.24.0       255.255.248.0     157.57.24.193     157.57.24.193     1157.57.24.193     255.255.255.255   127.0.0.1         127.0.0.1         1199.199.40.0      255.255.255.0     199.199.40.139    199.199.40.139    1199.199.40.139    255.255.255.255   127.0.0.1         127.0.0.1         1199.199.40.255    255.255.255.255   199.199.40.139    199.199.40.139    1224.0.0.0         224.0.0.0         157.57.24.193     157.57.24.193     1224.0.0.0         224.0.0.0         199.199.40.139    199.199.40.139    1255.255.255.255   255.255.255.255   199.199.40.139    199.199.40.139    1				


Check "Enable IP Routing" in the Advanced TCP/IP configuration to 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 199.199.40.139 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:
ARTICLE-ID: 157025
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 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 ServicePack 2 or later installed.

The route in this example means that to get to the 199.199.41.0 subnetwith a mask of 255.255.255.0 use gateway 199.199.40.1 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:
ARTICLE-ID: 141383
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:
ARTICLE-ID: 138793
TITLE : Multi-Protocol Router Installation and Configuration
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Article ID: 140859 - Last Review: 12/04/2015 12:33:14 - Revision: 2.2

Microsoft Windows 2000 Server, Microsoft Windows 2000 Advanced Server, Microsoft Windows 2000 Professional Edition, Microsoft Windows NT Workstation 3.5, Microsoft Windows NT Workstation 3.51, Microsoft Windows NT Workstation 4.0 Developer Edition, Microsoft Windows NT Server 3.5, Microsoft Windows NT Server 3.51, Microsoft Windows NT Server 4.0 Standard Edition, Microsoft BackOffice Small Business Server 4.0, Microsoft BackOffice Small Business Server 4.0a, Microsoft Windows for Workgroups 3.11, Microsoft Windows 95

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