Kerberos V5 uses port 88, which is the port
assigned by the IETF, for KDC requests. Kerberos V4 used port
750. If your users will need to get to any KDCs
outside your firewall, you will need to allow TCP and UDP requests on
port 88 for your users to get to off-site Kerberos V5
KDCs, and on port 750 for your users to get to
off-site Kerberos V4 KDCs.