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Determining link state using common network utilities - Windows

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Operating systems that support IP networking share a common set of utilities that can be used to determine a local hosts wired or wireless link state and IP connectivity. If a host has IP connectivity it can be concluded that that the wired or wireless link is in good working order. In Windows platforms, these utilities are...

ipconfig: Used to find the local IP configuration of a specific network interface.

ping: Used to test end-to-end IP connectivity between hosts.

arp: Manage the ARP cache on your local host.

Determining link state and IP connectivity example.

 

1. Open the Windows command line console (cmd.exe).

2. Enter the ipconfig command to find the IP address and default gateway of a specific network interface. In this example we find the IP configuration for the wireless LAN adapter.

C:\Users\someuser>ipconfig

Windows IP Configuration

Wireless LAN adapter Wireless Network Connection:

   Connection-specific DNS Suffix  . :

   Link-local IPv6 Address . . . . . : fe80::b44e:4877:76d5:db83%11

   IPv4 Address. . . . . . . . . . . : 192.168.128.249

   Subnet Mask . . . . . . . . . . . : 255.255.255.0

   Default Gateway . . . . . . . . . : 192.168.128.13. LAN communication is done a layer-2 of the OSI model. Hosts on the same LAN send data to each other using MAC addresses which is a layer-2 address. Every host builds a dynamic list of MAC address to IP address mappings in an ARP cache. This cache is populated by using the ARP protocol to discover which MAC addresses are associated to which IP addresses (layer-3) on the LAN. ARP is a detailed protocol and much has been left out here for the sake of brevity. Below is a brief description of ARP.
Each time a host needs to send data to another host on the LAN it checks its ARP cache. If a mapping exists the data is sent to the MAC address associated to the IP address of the destination host. If no mapping exists, the local host sends an ARP request broadcast to discover the MAC address associated to the specific IP address. The host that owns the IP address receives the request and responds with an ARP reply claiming ownership of the IP address and its associated MAC address. When the local host receives the ARP reply, it can update its ARP cache and send a frame directly to the destination host using its MAC address. Cache entries turn stale periodically so hosts frequently ARP to maintain an updated cache as old entries are aged out.


Use the arp command to clear the ARP cache and repopulate the list.

C:\Users\someuser>arp -D * 

4. Ping the IP address of the default gateway or another host on the LAN to test IP connectivity. Below is a continuous ping sent to the default gateway.

C:\Users\someuser>ping 192.168.128.1 -t

If the local link is up and the destination host is online and responding you will receive an ICMP reply for each ICMP request sent.


Pinging 192.168.128.1 with 32 bytes of data:

Reply from 192.168.128.1: bytes=32 time=4ms TTL=64

Reply from 192.168.128.1: bytes=32 time=1ms TTL=64

Reply from 192.168.128.1: bytes=32 time=1ms TTL=64

Reply from 192.168.128.1: bytes=32 time=1ms TTL=64

If timeouts are received between ICMP replies, there is most likely a cabling issue between the your host and destination host or the destination host is too busy to respond. If this is the case, try pinging a different host on the LAN. If the timeout go away then your local link is good and you need to troubleshoot the remote host.

Pinging 192.168.128.1 with 32 bytes of data:

Reply from 192.168.128.1: bytes=32 time=4ms TTL=64

Reply from 192.168.128.1: bytes=32 time=1ms TTL=64

Request timed out. 

Request timed out.

Reply from 192.168.128.1: bytes=32 time=1ms TTL=64

Reply from 192.168.128.1: bytes=32 time=1ms TTL=64

If the local link is up but the destination hosts firewall blocks ICMP you will receive ICMP timeouts. In this case, the destination host responded to your ARP requests which means your link state is good. Try disabling the firewall on the remote and run the ping again or try pinging a different LAN host. 

Pinging 192.168.128.1 with 32 bytes of data:

Request timed out. 

Request timed out. 

Request timed out. 

Request timed out. 

If the link is up but the remote host is unreachable you will receive Destination host unreachables. In this case, try pinging another host on the LAN. If pinging different hosts on the LAN also result in Destination host unreachable, it is a good chance your IP address is incorrectly configured or your uplink segment is good but the uplink segment connecting to the rest of the LAN is down. This is caused by a down port, VLAN mismatch, or bad cabling upstream.

Pinging 192.168.128.1 with 32 bytes of data:

Reply from 192.168.128.249: Destination host unreachable.

Reply from 192.168.128.249: Destination host unreachable.

Reply from 192.168.128.249: Destination host unreachable

Reply from 192.168.128.249: Destination host unreachable

If the local link is down you will receive a General failure. In this case make sure your adapter is enabled, re-seat your Ethernet cable and make sure local security software is not blocking the ping utility.

Pinging 192.168.128.1 with 32 bytes of data:

PING: transmit failed. General failure.

PING: transmit failed. General failure.

PING: transmit failed. General failure.

PING: transmit failed. General failure.

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