I am trying to understand how Cisco interfaces are named, so I am reading the document Interface and Line Numbers in Cisco 1800, 2800 and 3800 Series Routers

I am having some issues understanding the principles mainly because I don't understand the difference between a slot and a sub-slot.

Can anyone explain this to me?

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Jan 3, 2021 at 19:20

3 Answers 3


It will probably be easier to understand if you look at larger chassis (7600, 12k) and for example their SPA Interface Processor modules (SIPs) and SPA cards. Chassis have slots in which you can place designated hardware modules and those modules can have sub-slots. Keep in mind that some of this stuff can be integrated (probably the thing that is confusing you) or left for you to populate with desired hardware.

So in my example with SIP and SPA you will place SIP module in slot of the chassis, and then you place SPA cards inside sub-slots of the SIP.

In these configurations you will have interface names as:


In larger, high end routers, with IOS XR you can also see the chassis:


So the interface name describes the location of the interface from most significant part (slot in my example) to a least significant part (port in my example).


The x800 series you mentioned are the ISR G1 series routers. If you use the interface numbering model as interface type/slot/subslot/port, the built-in interfaces on the x800s are all slot 0. Thus, the first FastEthernet interface would be Fa0/0 (no subslot in this case). The x800s also have built-in WIC slots. So if a 2 port serial card is inserted into the first built-in WIC slot, the interfaces are number Se0/0/0 and Se0/0/1. If the second built-in WIC slot is populated with the same 2 port serial card, the numbering is Se0/1/0 and Se0/1/1.

The next slot available on an x800 would be the first network module slot. Thus, it won't be until you populate the first network module slot that you'll have anything other than "0" in the portion of the interface numbering on the x800 series.

In short, on the x800s, the numbering is:

    1. network module or "0" for built-in
    2. WIC (if applicable)
    3. port number.
  • cisco.com/image/gif/paws/62611/c2800_62611.pdf in this file, on page 5, there is a schematic of a switch.The Fa ports on the bottom start like this: 0/1/0. Shouldn't it be 0/0/0? I thought the numbering started with 0 on slot/subslot/port Aug 27, 2013 at 14:13
  • That's a good question. I don't have a real answer for you as I don't have this exact hardware setup. Sorry...
    – Ryan
    Sep 4, 2013 at 18:39

In order to overcome this limitation, Cisco 1800, Cisco 2800, and Cisco3800 series platforms now have a three-tiered interface numbering format (slot/subslot/port) for interfaces on WIC slots only.

I understand it like that: you can insert your WIC in the slot, and on the card itself there are subslots with ports. Juniper has similar naming convention if I remember correctly.

I'm not sure about this one though.

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