How to retain an ip address in sed while changing the netmask?

Question

I have the following problem: I want to change the netmask of large number of ip adresses in Debian's /etc/network/interfaces because I am going to expand the network. After changing it I will use screen ifdown ens18 && ifup ens18 to apply the changes. The substitution of the netmask works in my example, the problem is that I need to keep the 26 unchanged in the ip address. I have a sed line which changes all twenty-sixes to 25 but of course this is not what I want.

I have:

iface ens18 inet static
    address 195.26.153.26/26

I want:

iface ens18 inet static
    address 195.26.153.26/25

How can I achieve this?

Answer 1

To match only the netmask, match it with the separator:

sed sX/26X/25X

If you’re sure the netmask is at the end of a line, you can constrain the matches further:

sed 'sX/26$X/25X'