If you want or need to directly hook up a second computer to your primary one via Ethernet cable, what is the easiest way to find the IP address for the second one? Today’s SuperUser Q&A post provides some helpful advice for a frustrated reader.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

Photo courtesy of Keary O. (Flickr).

The Question

SuperUser reader Melebius wants to know how to find the IP address of a second computer directly connected to the first one by an Ethernet cable:

I have connected my primary computer to another one using a direct Ethernet cable connection. The second computer has no peripherals attached and I want to access it using RDP and SMB. The IP addresses are auto-configured, so it registers something in the range of 169.254.x.x.

I can wait until Windows recognizes the other computer or scan IP addresses, but both actions take a long and unpredictable amount of time. Is there a faster way to recognize the second computer at the other end of the Ethernet cable connection? I have considered making a broadcast “Ethernet ping” and reverse ARP, but I have been unable to find any instructions for this technique.

How do you find the IP address of a second computer directly connected to the first one by an Ethernet cable?

The Answer

SuperUser contributor grawity has the answer for us:

A broadcast IP ping might work. Not all systems answer to it, but some do when in 169.254 mode. Try ping (needs -b on Linux), or ping ff02::1 (needs ping6 on Linux).

Directly sending a name lookup (using nbtstat -a) might work (if it runs Windows and if you know the computer’s name).

The 169.254 auto-configuration involves sending some ARP probes with the host’s own address (you can see those in Wireshark).

“Ethernet ping” exists, but only works at the Ethernet level. It will not tell you anything about the IP (It is sometimes implemented in the NIC itself, but mostly not implemented at all).

“Reverse ARP” also exists, but is almost never actually implemented either. Its primary use was superseded by BOOTP and later DHCP.

Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

Akemi Iwaya
Akemi Iwaya has been part of the How-To Geek/LifeSavvy Media team since 2009. She has previously written under the pen name "Asian Angel" and was a Lifehacker intern before joining How-To Geek/LifeSavvy Media. She has been quoted as an authoritative source by ZDNet Worldwide.
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