I've recently been taking over an OpenStack environment that makes use of neutron networking with the use of GRE tunneling. We had a problem with network performance and I determined it to be an issue with the MTU setting in certain VM instances being set to the default 1500, whereas they really needed to be set lower, to say 1456 to allow for the encapsulation of the packets within packets over GRE.

While researching this I came upon these 2 sites:

On the second URL there was this diagram:


What does the F0/0 and F0/1 notation around the router in that diagram mean?

5 Answers 5


@3iron's answer gave me the hint that I needed. The notation is denoting that there are multiple NIC interfaces on the router in the diagram and it's making reference to this.

  • F0/0 - interface #1
  • F0/1 - interface #2

NOTE: The F indicates that the NIC/port is most likely a Fast Ethernet type of connection.

This diagram from the Cisco website shows them as well. Here's the referenced as Fa 0/1, which I believe is the more typical notation.


The other interesting take away is that the notation may sometimes include a 3rd digit to denote that the interface being referenced is not built into the Motherboard of the networking gear. The above diagram shows some examples of this, ie. T1 1/0/1. NOTE: The 3rd digit prefixes, and indicates which "slot" on the Motherboard a port is coming from.

Again the Cisco reference describes these slots like so:


I was also able to dig up this reference: Interface Nomenclature Guide of Router and Switch which describes the reference of F0/0 like so. Take note that there are 2 types of situations:

  • Fixed interface series router
  • Modular interface series router

This notation was originally used for fixed types of routers:

In fixed interface series router, the interface nomenclature is type slot_#/ port_#.

But with the advent of modular routers, that did not denote which addon card (WIC) was being referenced. So the notation was expanded:

  • New naming convention only for WIC slots (WAN Interface Card) is type slot_#/subslot_#/ port_#.
  • Ports installed directly on chassis still use classic convention that is type slot_#/ port_#.

Incidentally that same site had this table which describes the F0/0 notation like this:


Additional searches did turn up this URL: Standard Router Ports which had these types of ports listed:

  • Aux port: This auxiliary port is used to connect a modem to the router, which can then be used to remotely modify the configuration on the router.
  • Attachment Unit Interface (AUI) port: Before the WIC became a standard for providing expansion through an add-on port, the AUI allowed transceivers to be used, providing you with the ability to add various types of network connections, such as fiber or copper Ethernet connections.
  • Serial: Connects a modem or other serial device to allow a WAN network interface to be used on the router.
  • Ethernet/Fast Ethernet/Gigabit Ethernet: Standard network interfaces used to connect different network segments.
  • Console: Serial configuration port for command-line access to router management and configuration. Refer to Figure 3-1 to see the console port.
  • WAN Interface Card (WIC) port: Because a wide variety of WAN connectivity options are available (for example, T1, ISDN, ADSL), you can use this port to add different interfaces to a standard router.
  • Hardware WAN Interface Card (HWIC) port: With the integration of services into routers, the WIC interface became too limiting. The HWIC interface was created to support a wider variety of hardware expansion options, such as switches and service cards. This port is backward compatible with most older WIC hardware.

As well as this URL: Different types of interfaces in a Cisco Router:

  • Ethernet - Ethernet is typically Ethernet IEEE 802.3 standard based physical interface, which operates at 10 Mbps speed. The media standard used is 10BaseT.
  • Fast Ethernet - Fast Ethernet is typically Ethernet IEEE 802.3u standard based physical interface which operates at 100 Mbps speed. The media standard used is 100BaseT.
  • Gigabit Ethernet - Gigabit Ethernet is typically Ethernet IEEE 802.3ab standard based physical interface which operates at 1000 Mbps speed. The media standard used is 1000BASE-T
  • Serial - Serial interfaces are typically used for WAN connections from ISP (Internet Service Providers) for connectivity types like Frame Relay, T1, T3, etc.
  • FDDI Fiber Distributed Data Interface - FDDI networks operates at 100 Mbps speed and uses a token-passing mechanism to prevent collisions.
  • Token Ring - Token Ring interfaces can operate at either 4 Mbps or 16 Mbps. In Token Ring networks a token is passed around the network (configured in ring topology), allowing the owner of the token to transmit a frame, to avoid collision.Token Ring networks vanished from networking industry long way back. New Cisco routers do not have a Token Ring interface.

Note: Only 10Mbps Ethernet interface has a name "Ethernet" in a Cisco Router. A 100Mbps Ethernet interface is called a "FastEthernet" interface and a 1000Mbps Ethernet interface is called a "GigabitEthernet" interface.

That last bit is a bit more telling. Since Ethernet would historically been used for 10MBps interfaces, the "F" in my "F0/0" & "F0/1" diagram would seem to imply that the interface is a FastEthernet capable connection (10/100MBps).

However, more recently, Ethernet can also be used interchangeably on Gigabit Ethernet as well as 10G Ethernet ports, so you need to pay special attention to the actual equipment that's been referenced by a diagram.


  • 1
    The "F" doesn't imply that the connection is 100 Mb; it means that the interface is a FastEthernet (100 Mb) interface, but, in most cases, can connect at 10 or 100 Mb so you can't assume that the connection is actually running at 100 Mb without looking at it in the router.
    – Ron Maupin
    Commented Jun 5, 2015 at 14:35
  • @RonMaupin - fixed.
    – slm
    Commented Jun 5, 2015 at 14:40
  • 3
    Figured since you put so much work into this, I should muddy it up with more facts. Not only can routers have a "3rd digit" but stacking switches will generally have them as well with the additional number specifying the switch number in the stack. Also, while "Ethernet" was originally used to specify 10Base-T, it is now also used on newer platforms like the Nexus to indicate an Ethernet port even if it is Gi or 10G.
    – YLearn
    Commented Jun 5, 2015 at 16:32
  • 1
    You have different ways, depending on the hardware, of denoting the port name. You could have what you are looking at here: <slot>/<port>. Also common on chassis-based or stacks is <slot>/<module>/<port>. The stack uses the <slot> to denote the physical switch. For example, the 3750X uses G1/0/1 for the first built-in port, but G1/1/1 or T1/1/1 for the first port in the NM module, on the first or only switch. A 2nd switch would use G2/0/1 and G2/1/1 (T2/1/1).
    – Ron Maupin
    Commented Jun 5, 2015 at 17:30
  • 2
    @slm, to further clarfiy, since a switch stack is managed as a single unit (typically connected by stacking cables and not "traditional" network interfaces), in many diagrams they are represented as a single switch, although I have seen them represented as individual devices as well.
    – YLearn
    Commented Jun 5, 2015 at 18:54

F0/0 and F0/1 look to be router interface notations to highlight a physical connection into the router from the two respective hosts.

Typically they could be:

  • Fa: FastEthernet
  • Gi: GigabitEthernet
  • Ten: Ten GigabitEthernet
  • 3
    score for 3iron on the info that help @slm... but I think he should accept his own answer now that he put massive effort into writing us a router interface ID 101 page :^D Commented Jun 5, 2015 at 19:26

Actual format is Interface card/slot/port numberes

Some devices (like 2500's) it's just a port number (Serial0)

In modular devices, you may have a slot/port number (Serial 0/0)

In larger modular devices, you'll get card/slot/port numberes (Serial 0/0/0)


Short Answer: F0/0 and F0/1 are the "names" of the interfaces in the examples. Since they're names, it can be dangerous to infer anything from them (see below).

Longer Answer: This naming nomenclature is common in Cisco switches and as such has been cloned by many other switch providers to prevent having to train engineers on two different naming methodologies. This means that both the vendor and when the device was made may impact how the term is applied. In some devices it may indicate expansion card and port capabilities while others just hardcode names.

If you are going to try and determine actual product capabilities it would be best to use something like Link Layer Discovery Protocol (LLDP) or something similar which is a little more rigorously defined and based on current operating conditions of your network.


The first 0 represents the interface card in a router. Many routers support multiple interface cards, and each card can have multiple connections. So the second number represents that actual interface on a given interface card. So 0/1 refers to the number 1 interface on the number 0 card installed in the router. If the router has other cards, you could see interface numbers like 1/1.

source : Head First Networking, May 2009 First Edition (page 254)

  • If this answer is correct, it was the most helpful to me. Do you have a source for this information? What does "Page 292" refer to? Commented Sep 14, 2017 at 19:18

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