23 March 2017

Looking for Ethernet segment peers

A brief set of instructions for checking whether there are any other Ethernet hosts on the same segment as you.

Assumptions

  • a computer running Microsoft Windows
  • an Ethernet IPv4 network
  • nothing prevents you from sending ping requests
  • nothing prevents any other hosts on your segment from receiving ping requests (this is a likely point of failure, since firewalls often reject or discard ping packets)

Instructions

  1. Go to your Windows box. Find out your own IP address and subnet mask; this is done by typing ipconfig. Example: IP address 10.10.10.10, net mask 255.255.255.0.
  2. Look up the broadcast address for your network in RFC 1878. Continuing the above example, the broadcast address would be 10.10.10.255.
  3. Ping the broadcast address by typing e.g. ping 10.10.10.255 (using the broadcast address you found above). This should make all hosts on your segment advertise their MAC addresses to you.
  4. View your ARP cache by typing arp -a. You should see the IP address of your router. Any other IP addresses are other hosts on your segment.