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Help understanding what's happening

I noticed this today while labbing at work, and now have replicated it at home. I'm certain I'm missing something obvious, but for the life of me can't figure out what it is. When I ping 10.1.x.x I always get a response, despite there not being any such hosts on the network.

My lab at work (standalone) is on a network. No routes to any other networks. Pinging 10.1.x.x or 10.2.x.x gets responses from those addresses.

I thought it must be something with my subnetting, but investing at home led me to the same results - on a 192.168.0/24 network. A traceroute takes me through my gateway then a couple of 100.127.x.x hosts, so now I'm wondering if it has to do something with NAT and my ISP (same provider at both locations).

Even though I'm just starting my work towards CCNA, I'm feeling dense for not being able to figure this out. It's not causing any problems, which leads me to believe even more it must be some fundamental and expected thing I am overlooking. Can someone please point me in the right direction?

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level 1
CCNA R&S / Sec - Cyber in progress3 points · 6 months ago

do you have the same ISP as work?

I would not be surprised if they forgot to block RFC 1918 space at either the customer edge or the provider edge.

I have seen other anomolies. Once someone plugged a hub up outside our router and used it to connect to the ISP and also our main switch stack. We got a call from a local garage way out in Alabama (same town as our remote site) saying a welcome message popped up. Turns out the hub was allowing the DHCP traffic to go upstream and he was working off of our network connection.

level 2
Original Poster3 points · 6 months ago

Same ISP. Hearing you say it, I bet they just aren't blocking it at the edge, and these are their hosts (servers, management, etc...).

level 1
⚡️Net+✨3 points · 6 months ago

A traceroute takes me through my gateway then a couple of 100.127.x.x hosts,

That's cool, you're ISP should be filtering private addresses but you're going through their network.

Who's your ISP?

level 2
Original Poster1 point · 6 months ago

Right? I wonder how long it'll go before someone sees it. Cox is my ISP.

level 3
CCNA R&S3 points · 6 months ago

Ahh! Cox is the only ISP that I know of that definitely does this, although I'm certain that there are a few others that do as well.

This is actually documented fairly well on the internet! It's definitely a Cox thing.

The 100.127.x.x hosts are actually part of CGN (Carrier-Grade NAT), as that falls within the reserved range for CGN ( –

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