Kdjykuj

I have a usb wireless adapter which is eth1. When it is plugged in it automatically gets an ip address of 192.168.8.100/24 and routes automatically added:

ip a:

39: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000

    link/ether 0c:5b:8f:27:9a:64 brd ff:ff:ff:ff:ff:ff

    inet 192.168.8.100/24 brd 192.168.8.255 scope global dynamic noprefixroute eth1

       valid_lft 85920sec preferred_lft 85920sec

    inet6 fe80::640:311a:de17:1163/64 scope link noprefixroute 

       valid_lft forever preferred_lft forever

ip route:

default via 192.168.8.1 dev eth1 proto dhcp metric 100

192.168.8.0/24 dev eth1 proto kernel scope link src 192.168.8.100 metric 100

The internet is accessible, but if I change the ip address to 192.168.2.100/24 and add routes which are identical to the ones added but with the new ip address and a higher metric, it doesn't work:

ip address del 192.168.8.100/24 dev eth1

ip address add 192.168.2.100/24 dev eth1

ip route add default via 192.168.2.1 dev eth1 proto static metric 99

So now the the interface and routes look like this:

ip a:

39: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000

    link/ether 0c:5b:8f:27:9a:64 brd ff:ff:ff:ff:ff:ff

    inet 192.168.2.100/24 scope global eth1

       valid_lft forever preferred_lft forever

    inet6 fe80::640:311a:de17:1163/64 scope link noprefixroute 

       valid_lft forever preferred_lft forever

ip route:

default via 192.168.2.1 dev eth1 proto static metric 99 

default via 192.168.8.1 dev eth1 proto dhcp metric 100

192.168.2.0/24 dev eth1 proto kernel scope link src 192.168.2.100 

192.168.8.0/24 dev eth1 proto kernel scope link src 192.168.8.100 metric 100

Why does it not work?

If 192.168.2.1 is not setup in the access point side, obviously routing wont work. The reminder of the answer is written supposing it is.

The question is assuming an authenticated Wifi medium does behave as a your regular shared Ethernet medium by default; it does not.

In an authenticated Wifi network, basically, in simple terms, you can think it emulates an ethernet shared medium after you authenticate, for security sake. Basically, after authentication you are given an IP address that might be tied to the authentication (depending on several factors).

If you do change that IP address, the association might be lost, and you have to (re)start the Wifi authentication process all over again.

There also might be firewalling rules in place that prevent you from the clients changing for another network.

PS for more advanced users, both Wifi and Ethernet can have authenticated or non-authenticated setups. I am just trying to lay it down in simple terms for the benefit of the OP.