The IP address 172.21.91.172 : Class B networks
Secure IP address This IP is only local, it is not accessible from outside the network
Rating: 5
This means that the IP address is not accessible from the Internet, it is local and only accessible within the network. Everyone has the same local address (172.16.0.0 - 172.31.255.255).
Each device (laptop, printer, scanner, phone, surveillance camera, tablet, ...) has a unique local IP address assigned by the adsl box that allows the box to identify the different devices to within a local network and send the right product at the right flow.
Specifically, the adsl box has a public IP address in your case, which connects to the Internet and the box assigns a local address to all devices to identify them.
The IP address 172.21.91.172 detail
The IP address 172.21.91.172 (IPv4 Class B networks) belonging to the network 172.16.0.0 - 172.31.255.255 is written in long version -1407886420.
Color obtained mathematically from the IP address: Dark fuchsia
Addresses on the same network : 172.21.91.0 172.21.91.1 172.21.91.2 172.21.91.3 172.21.91.4 172.21.91.5 172.21.91.6 172.21.91.7 172.21.91.8 172.21.91.9 172.21.91.10 172.21.91.20 172.21.91.30 172.21.91.40 172.21.91.50 172.21.91.60 172.21.91.70 172.21.91.80 172.21.91.90 172.21.91.100 172.21.91.200 172.21.91.254 172.21.91.255