[vnet] fix: mask default IP route on windows

[nklaassen]

This commit updates the netsh interface ip set address command used to set the v4 IP address for the TUN interface on Windows to include a mask for the correct CIDR range. Without the mask, Windows assigns a default route for the nearest /8, /16, or /24 range to the address. With the default range of 100.64.0.0/10, this was causing a route for 100.0.0.0/8 to be assigned to VNet, which should not handle all of that range.

Before this change the assigned routes looked like this:

IPv4 Route Table
===========================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
          0.0.0.0          0.0.0.0     192.168.50.1    192.168.50.16     30
        100.0.0.0        255.0.0.0         On-link        100.64.0.1    261
       100.64.0.0      255.192.0.0       100.64.0.1       100.64.0.1      6
       100.64.0.1  255.255.255.255         On-link        100.64.0.1    261

With the fix it looks like

IPv4 Route Table
===========================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
          0.0.0.0          0.0.0.0     192.168.50.1    192.168.50.16     30
       100.64.0.0      255.192.0.0       100.64.0.1       100.64.0.1      6
       100.64.0.1  255.255.255.255         On-link        100.64.0.1    261

note that the extra route for 100.0.0.0 with a mask of 255.0.0.0 has been removed.

This is not an issue on darwin.

changelog: fixed a default IPv4 route assigned to the VNet interface on Windows

[zmb3]

The comment below still refers to the original ipnet variable name. Please update.

[backport-bot-workflows]

@nklaassen See the table below for backport results.

Branch	Result
branch/v17	Failed
branch/v18	Failed