Resolving ARP cache problems

nics

ARP stands for Address Resolution Protocol and at its simplest defnition, it allows network software (ex. a mapped network drive) to talk to network hardware (ex. the network card on a remote file server).  If ARP is not properly functioning on your computer, you won’t be able to view web pages, connect to network systems, etc.  This article briefly explains the purpose of ARP and how to resolve common ARP related problems.

NOTE: I am no network guru (I am a Windows automating, desktop engineering, mispelling wizard).  I will try to provide my high level understanding of network communications to help you understand what ARP and the ARP Cache is and why you need it.  If you spot anything incorrectly represented below, let me know and I will update.  Thanks. 

Networking Overview
Networked computers have devices called network cards which allow them to connect to a network.  For example, a desktop computer may have a network card where you connect a network wire between it and your router/cable modem; while a laptop may use it’s wireless network card to wirelessly connect to a wireless access point.  Despite if the card is wired or wireless, every single card has a unique identifier called a MAC address which is used to talk to other network devices (think of this as the network telephone number). 

It would be a nightmare to expect people to remember MAC addresses to talk to remote computers, so instead people are just expected to know names (ex. jesseweb.com).  After you initiate a connection by name (ex. START | RUN | COMPUTERNAME), your computer will first make a DNS (Domain Name System) request to a DNS server to look up and retrieve the IP address (sets of numbers that uniquely identify a remote system) associated with the remote system’s name.  NOTE: All properly configured networked computers are typically assigned DNS servers manually (i.e. your network administrator set this on your computer before they gave it to you) or dynamically (ex. assigned by your cable modem). 

Your computer still needs to know the MAC address of the system you are trying to talk to, so it will  send an ARP broadcast (think of this as your computer shouting) asking the computers on your immediate local network for their IP addresses and associated MAC addresses.  Your computer will store the information returned from each broadcast reply in a local table called the ARP cache (or ARP table).  By storing/caching this information, your computer will not have to shout for this information the next time – thus speeding up subsequent calls.  If the IP address your computer is looking for is in the ARP cache, your computer can now start talking to the remote computer.  If it is not, your computer will comunicate with its assigned gateway (assigned similarly as the DNS server is assigned), asking it t connect your computer with the local computer (think of the gateway as a telephone operator).  The call may pass from one gateway to another (known as hops), until eventually your computer is connected with the desired remote system.

Resolving ARP Cache Problems
If you can’t properly use the ARP cache for one reason or another (ex ARP Cache corruption), you won’t be able to talk to computers on the network (ex. can’t get on the Internet).  Below are suggested steps to resolving ARP Cache related problems:

  • Repair your network connection – Starting with XP, you can right click on a network connection and chose an option to repair it.  This should be your first step in resolving any network related problems.
    • If you receive the following error after repairing the connection, see the Routing and Remote Services bullet below:

Windows could not finish repairing the problem because the following operation could not be completed:
Clearing the ARP cache
For assistance, contact the person who manages your network

  • Clear the ARP cache – START | RUN | netsh interface ip delete arpcache
  • Routing and Remote Services – Disable this Windows service and see if your problem goes away.  If so, there is likely a problem with your Routing and Remote Services configuration (ex. you have the Routing and Remote Services firewall enabled on your one and only network card, preventing incoming/outgoing connections) .
  • Review the ARP Cache – START | RUN | CMD | ARP – a

Jesse Torres

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