From a message by xDSLMan:

For firmware versions 3.1.0 to 3.5.x (with the exception of 3.5.15)-

172.16.0.1 (netmask 255.255.0.0)
with 172.16.1.1 to 172.16.1.32 reserved for static and 172.16.1.33 and up for dhcp.

other selectable ranges:

10.0.0.1 (netmask 255.255.0.0 ) as ip on the gateway
10.0.1.1 and 10.0.1.32 for static and 10.0.1.33 and up for dhcp.

192.168.0.1 (netmask 255.255.0.0) for the gateway
192.168.1.1 to 192.168.1.32 for static and 192.168.1.33 on up for dhcp.

This changed in firmware versions 3.7.x to-

192.168.1.254 (netmask 255.255.255.0) as gateway's default ip/subnet.
192.168.1.1 to 192.168.1.63 available for static and 192.168.1.64 on up as dhcp range.

The other two possibilities 10.x and 172.16.x remained unchanged.

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by Riss_Centaur