> ## Documentation Index
> Fetch the complete documentation index at: https://developers.telnyx.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Source Address

> Functions and hosts on a Network see traffic from a Tailscale Subnet Router's customer_address, never the device's own address.

Traffic from devices behind the router enters the [*Network*](/docs/network/overview) from the router's `customer_address`, one address per router from `172.27.254.0/24`. A function or host on the *Network* sees that address as the source, never the device's own address.

A device behind the router calls a function at `172.27.0.4` that returns the source address it sees. The function runs on Node.js, which prints IPv4 sources in IPv6-mapped form:

```bash Device theme={null}
curl -s http://172.27.0.4:8080/ | jq -r .caller
```

```text Output theme={null}
::ffff:172.27.254.1
```

It then calls a host at `172.27.0.3` that echoes the source address:

```bash Device theme={null}
curl -s http://172.27.0.3:8787/
```

```text Output theme={null}
hello from docs-host, caller 172.27.254.1
```

Both see `172.27.254.1`, the router's `customer_address`.