what is a good invalid IP address to use for unit tests?

There's 3 private IP blocks you can use for such things:

10/8 (10.0.0.0 -> 10.255.255.255) (an old school Class A netblock)

172.16/12 (172.16.0.0 -> 172.131.255.255

196.168/16 (192.168.0.0 -> 192.168.255.255) (an old school Class B netblock)

According to RFC 5737:

The blocks 192.0.2.0/24 (TEST-NET-1), 198.51.100.0/24 (TEST-NET-2), and 203.0.113.0/24 (TEST-NET-3) are provided for use in documentation.

This means you can use pick an IP address from these ranges:

• 192.0.2.0 - 192.0.2.255
• 198.51.100.0 - 198.51.100.255
• 203.0.113.0 - 203.0.113.255

If you're looking for a truly invalid IP address (as opposed to an unrouteable one), you can take advantage of the fact that the first byte of a Class A address cannot be 0.

For example:

0.42.42.42

The answer from Jonathan (and it's comments) are good for IPv4

If your tests support IPv6 it has an explicit black hole you can use: 0100::/64 as defined in RFC 6666

It's tempting to use 254.254.254.254 since it's easy to remember, but on some platforms it gives a immediate "transmit failed. General Failure" rather than an actual timeout which may be an invalid unit test.