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I have a requirement to regularly produce spreadsheet showing Cisco router interface information, in particular bandwidths of each interface and IP addresses. I have access to a network management tool which I can pull reports from so the information is there, I just need to understand it and present it logically

So one of the requirements is to show LAN and WAN side IP addresses in the spreadsheet under their own column. I understand the difference between these in a home network context – LAN side is the local IP that other devices see and the WAN side is the address that the internet sees. However in this context, I cannot seem to separate which is which..

This is what I have to work with (IPs and interface descriptions have been removed) for example for each interface

Device name, Interface Name Interface Description IP Address Speed Type Hostname1 FastEthernet0/0/0 (Fa0/0/0) Example LAN 2 MBit/s (F) ethernetCsmacd

How do I look at the IP addresses and work out whether the IP relates to the LAN or WAN side?

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  • 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 provide and accept your own answer.
    – Ron Maupin ♦
    Commented Aug 12, 2017 at 18:41

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First, it is important to understand that the exact particulars of what is LAN or WAN in any given network are subjective for that network. General LAN\WAN definitions can be conjured up but specific network designations will vary from network to network. Next, it is impossible to look solely at an IPv4 address and state for certain whether or not it belongs to the LAN or WAN. One needs to understand more about the addressing in a particular network in order to know for sure. This of course excludes the RFC1918 private IPv4 space which I personally would always consider as LAN.

Further, the delineation between LAN and WAN is not as clear as it used to be. Loosely speaking, the traditional demarcation between the two was private and public meaning the stuff "we" control (private - LAN) and stuff controlled by someone else (service provider - WAN). This delineation is still fairly correct today however the issue has been clouded somewhat by the advent of VLANs, VPLS, MPLS, MetroEthernet, etc.

Therefore, in today's vernacular, it seems safe to say that any address that is upstream and outside of your network - especially IPs that are used to directly connect your network to the Internet - are likely WAN and all other addresses within your network could safely be deemed LAN.

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Usually the WAN IP address will be a publicly routable IP address while the LAN will be a private IP address.

Note that it is not always the case.

Private IP address are defined in RFC1918 and are:

 10.0.0.0        -   10.255.255.255  (10/8 prefix)
 172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
 192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

If you get one IP in one of those blocks and another one which is not then you are good, otherwise we can't tell you.

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  • Thanks - So its as simple as public or private?
    – user105971
    Commented Jan 12, 2017 at 13:19
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    @user105971, maybe. You need to understand a lot more about how the network is connected. In my company, we own a lot of public IP address space, and much of it is on what you would term the LAN side. The WAN side is connected to your ISP(s). This is why you need to maintain network diagrams.
    – Ron Maupin ♦
    Commented Jan 12, 2017 at 15:43
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Actually if you are looking at a company network then the private versus private addresses does not work. You cannot tell by looking at an IP address is LAN or WAN just by the address. However generally WAN interfaces have a subnet mask of 255.255.255.252.

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You will need to ask a person that knows your network. Here's an examples.

192.168.3.0 255.255.255.0 This could be my LAN address internal to this site.

192.168.10.0 255.255.255.0 This looks like LAN but is a WAN address used for a remote network in this case. It could have been local LAN also. Just depends on the network design.

69.75.43.125 255.255.255.252 although we all know this should be a WAN this could be a LAN link somebody set up not knowing any better or just being... I'll be nice and say creative.

That's why you can't tell by the address. You will need to ask someone who knows or start using commands like show cdp neighbor in Cisco or show lldp info remote on HP (if LLDP is enabled). NOTE... Some HP has CDP and Cisco can use LLDP. From that you can try to figure out where the equipment that shows in these commands are located if they are named properly or by tracing ports. Traceroute could help as well as you learn where some networks are in your organization. Easiest way is ask your network guy or I. T. provider.

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