3Com MSR 50-40 ROUTER. Installation Manual page 54

The interfaces are represented by interface-type X/Y, where interface-type can be serial,
asynchronous, Ethernet, ATM, and so on; X specifies the slot number; and Y specifies the interface
sequence.
Different interfaces on an interface module share the same slot number X.
For every interface, Y starts from 0, indicating the interface sequence on the interface module, from
left to right.
If you install an FIC-1FE and an FIC-2FE respectively in SLOT5 and SLOT6 on the MSR 50-40, the
Ethernet interfaces are numbered as follows:
The fixed GE interfaces are Gigabit Ethernet 0/0 and Gigabit Ethernet 0/1;
The Ethernet interface on the FIC-1FE is Ethernet 5/0;
The Ethernet interfaces on the FIC-2FE are Ethernet 6/0 and Ethernet 6/1.
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