Level 3 DNS Servers:
4.2.2.1
4.2.2.2
4.2.2.3
4.2.2.4
4.2.2.5
4.2.2.6
The odd ones (.1, .3, .5) will correctly reply with NXDOMAIN for FQDNs which don't exist. The even ones (.2, .4, .6) will instead resolve to two "SearchGuide" IPs:
~]$ dig @4.2.2.2 domain-i-just-made-up.fake +shortAny hosts that may have been configured to use these Level 3 DNS servers will have some interesting outgoing connections when trying to connect to internal hosts that aren't externally resolvable. Hopefully this will save a few minutes when investigated unusual connections to 198.105.244.11 or 198.105.254.11.
198.105.244.11
198.105.254.11