• Suppose the world is partitioned into n different Kerberos realms. It
might be the case that principals in one realm need to authenticate
principals in another realm.
• KDC in realm B can be registered as a principal in realm A. This
allows users in realm A to access realm B's KDC as if it were any
other resource in realm A.
• It doesn't work in Kerberos V4 to start in realm A, get a ticket to
realm B, and from there get a ticket to realm C. In order for a
principal in realm A to talk to a principal in realm C, C's KDC has to
be registered as a principal in realm A.
• Kerberos V4 deliberately prevents access through a chain of KDCs.
If it didn't, then a rogue KDC could impersonate not only its own
users, but those of any other realm, by claiming to be the
penultimate KDC in a chain.