A string of characters whose exact syntax is that for a DECnet or
 X25 circuit identification.

 For DDCMP circuits, the circuit identification is in the
 form dev-c-u.t, for example, DMP-0.1.

 For Ethernet circuits, the circuit identification is a string in
 the form dev-c, for example, UNA-0.

 For FDDI circuits, the circuit identification is a string in the
 form dev-c, for example, MFA-0.

 For X25 circuits, the circuit identification is a string of up
 to 15 characters beginning with "X25-".