Is public IP X advertised to the Internet?

I work with customers who often run into asymmetrical routing issues from their on-premises networks because they are advertising a public IP to both the Internet and to a cloud provider. I’ve looked at numerous looking glasses/forums but can’t find one that tells me if a specific IP is advertised on the Internet. Any ideas?

Thank you

Hello Brian,

On the Internet, there isn’t a single complete “source of truth”. The available routes depend on where you are in the network. A looking glass server with a full Internet routing table (940k routes at this time of writing) is your best bet, some of your customer routes are probably aggregated though.

Give https://lg.he.net/ a try. You can query multiple routers. For example, here I checked IP address 81.204.123.16:

This IP address belongs to my ISP (KPN - The Netherlands). This is a large ISP that advertises a /14 network. Such a large /14 block shows up everywhere. I checked a router in Germany, the USA, and New Zealand.

Next, I looked up a /29 route in the route-views.optus.net.au looking glass server (Australia):

*> 59.154.24.24/29  203.202.143.33                         0 7474 i
*                   202.139.124.130         20             0 7474 i
*                   203.13.132.7            10             0 7474 i
*                   203.202.143.34                         0 7474 i
*                   192.65.89.161            1             0 7474 i

This is a small /29 network so it’s likely to get aggregated somewhere: When I look up 59.154.24.24 on lg.he.net, here’s what I see:

It shows up on these routers in Germany, the USA, and New Zealand but as a /16 route. Somewhere, it got aggregated.

In your case, you probably also have customers with small networks. Your best bet is to check multiple looking glass servers, close to your customer. There might be other websites similar to https://lg.he.net/ so you can look this up quickly on multiple routers simultaneously.

I hope this helps!

Rene

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