ARP Request for Nonlocal Target Hosts on Same Physical Net (150577)


The information in this article applies to:

  • Microsoft Windows NT Workstation 3.5
  • Microsoft Windows NT Workstation 3.51
  • Microsoft Windows NT Server 3.5
  • Microsoft Windows NT Server 3.51
  • Microsoft TCP/IP-32 for Windows for Workgroups 3.11
  • Microsoft TCP/IP-32 for Windows for Workgroups 3.11a
  • Microsoft TCP/IP-32 for Windows for Workgroups 3.11b
This article was previously published under Q150577

SUMMARY

If multiple logical subnets are present on the same physical network (with no routers between the subnets), communication between hosts from different subnets might not be possible. The packets destined for nonlocal subnets will be sent to the router (if configured for one) or dropped (if no default gateway is present). The Address Resolution Protocol (ARP) is used by the host to find the physical address (MAC address) of a target host. An ARP request packet is broadcast on the local subnet if the target host subnet address matches the host's subnet address. If the subnet addresses do not match, then ARP resolves the MAC addresses for the router.

MORE INFORMATION

The following information is extracted from a Microsoft whitepaper on TCP/IP.

When running multiple logical subnets on the same physical network, the following command can be used to tell IP to treat all subnets as local and to use ARP directly for the destination: 

   route add 0.0.0.0 MASK 0.0.0.0 <my local ip address>


Thus, packets destined for "non-local" subnets will be transmitted directly onto the local media instead of being sent to a router. In essence, the local interface card can be designated as the default gateway. This might be useful where several class "C" networks are being used on one physical network with no router to the outside world.
Modification Type:MajorLast Reviewed:12/16/2003
Keywords:KB150577
©2003 Microsoft Corporation. All rights reserved.