AS21508: the COMCAST-21508 network

AS21508 is an internet Autonomous System operated by COMCAST-21508 (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 AS21508?

An Autonomous System (ASN) is a block of networks under a single control with its own routing policy on the internet. The number AS21508 was assigned to COMCAST-21508 by an RIR (regional internet registry). All traffic entering or leaving that network is announced under this number.

Reach on the internet

AS21508 currently announces 135 IPv4 block(s), totaling about 689.2 thousand addresses (689,152 in all), registered in the United States. Services from COMCAST-21508 are reached through these addresses.

What it means to see AS21508 on your network

When you investigate where a connection from your computer or phone comes from (or goes to), you may find AS21508. That means the other end sits on the COMCAST-21508 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.