Apple - How to find IP address of my other Mac?

Your computers should be accessible as computer-name.local, e.g. Kens-Computer.local or something like that, so depending on what you're trying to do, you might be able to just use that name rather than the IP.

So you can just type vnc://Kens-Computer.local in the 'Connect to' window. You can see the computer's network name if you go to the Sharing preference pane in System Preferences. It's at the top of the page, with a message like "Computers on your local network can access your computer at: Kens-Computer.local"

Make sure you include the ".local" suffix to get the Bonjour IP, otherwise your DNS look-up might auto-append some other hostname suffix (depending on your network/DHCP configuration) or simply fail to find the right host.

If you really want to get the IP though, here's a couple of options:

Bonjour / dns-sd

You can use Bonjour directly via commands like the DNS Service Discovery tool (dns-sd). Try the following in the Terminal application:

% dns-sd -q computer-name
DATE: ---Tue 09 Oct 2012---
18:13:39.209  ...STARTING...
Timestamp     A/R Flags if Name                             T   C Rdata
18:13:39.210  Add     2  4 Computer-Name.local.             1   1 10.1.1.141

The dns-sd tool can do quite a bit, e.g. if you want to list all local VNC services (at least the ones advertised via Bonjour). VNC services are advertised as "_rfb" in mDNS/Bonjour:

% dns-sd -B _rfb local

Ping

Or you can just ping it, which will show the IP in the output (and is just using Bonjour/mDNS to find the IP).

% ping computer-name.local
PING computer-name.local (10.1.1.141): 56 data bytes
64 bytes from 10.1.1.141: icmp_seq=0 ttl=64 time=0.066 ms

Open Terminal and type: arp -a it will list all the computers on the network and (usually) the host names.

You can login to the router and look for DHCP Table. It will list all connected devices along with their MAC and IP addresses.

But to avoid this issue altogether, why don't you setup DHCP Reservations through your router? Through that, you can assign a certain MAC address to a specific IP. That way your DHCP IPs remain constant.

The steps necessary to achieve that will depend on the router brand and model.