AS21100: the ITLDC-EU network

AS21100 is an internet Autonomous System operated by ITLDC-EU (the United States). Here is what that means and why this number may show up when you investigate a connection on your network.

What is AS21100?

An Autonomous System (ASN) is a block of networks under a single control with its own routing policy on the internet. The number AS21100 was assigned to ITLDC-EU by an RIR (regional internet registry). All traffic entering or leaving that network is announced under this number.

Reach on the internet

AS21100 currently announces 28 IPv4 block(s), totaling about 13.8 thousand addresses (13,824 in all), registered in the United States. Services from ITLDC-EU are reached through these addresses.

What it means to see AS21100 on your network

When you investigate where a connection from your computer or phone comes from (or goes to), you may find AS21100. That means the other end sits on the ITLDC-EU network (the United States). It can be legitimate — a cloud service, an ISP, a CDN — or an unexpected connection worth checking.

How to see every connection and network your devices reach

Tracing one connection's ASN at a time is tedious. Sentinela, by PC Monitor, maps the devices on your network, shows what is exposed to the internet and helps flag suspicious connections and devices — running on your Windows. Meet Sentinela.