Get Subnet mask in Linux using bash

Question

I am using bash to get the IP address of my machine with that script:

_MyGW="$( ip route get 8.8.8.8 | awk 'N=3 {print $N}' )"

And now I am trying to get the Subnet Mask in this type:

192.168.1.0/24 

But I have no idea how can I do that.

Lewison

8.8.8.8 via 192.168.1.1 dev eth0 src 192.168.1.5 ( the IPs are not real - only for example ) – Lewison

So /24 is not in your output of ip command but you want to get it in final output? – Lubin

Yes, but it's not necessary to be with ip command . – Lewison

This command just print my ip and in the end add /24. ip route' output is: 192.168.1.0/24 ` where the last number before ` is 0`. – Lewison

Please clarify how some arbitrary command is supposed to guess what subnet you want to use. An IP address by itself has absolutely zero information about what subnet mask should be used. The only possible exception would be those subnets reserved for private use (such as 192.168.0.0/16), but even then the subnet does not have to be /16. It could be /24, as in your example, or even anything else from 16-32. – Overtly

In other words, ip route get does not provide the information needed to print the proper subnet information. – Overtly

See here for conversion functions. – Unessential

Answer 1

there are couple of ways to achieve this:

first: to print the mask in format 255.255.255.0, you can use this:

/sbin/ifconfig wlan0 | awk '/Mask:/{ print $4;} '

second: we can use ip command to get the mask in format 192.168.1.1/24

ip -o -f inet addr show | awk '/scope global/ {print $4}'

Gopak

Vishal, that works. Thank you. But I have a questions. I changed wlan0 with eth0 in my case but when I start the first line of your answer, I don't have any input. What actually should be do this ? – Lewison

first make sure that you have eth0 connection using ifconfig command, if you have then it should print, it worked in my case. :) – Gopak

Yes, I have a connection. But I understand what should be doing. Actually I don't have need from this output. – Lewison

precisely you can check that from the second command, just add $2: ip -o -f inet addr show | awk '/scope global/ {print $2,$4}' – Gopak

Yes, this is very helpful for me, because I have more then one interface. And actually I am trying to make a script which delete this networks with ip route command. – Lewison

One more question, is ti possible to change the last number of my ip before /xx to be 0. Like the example in question 192.168.1.0/24. – Lewison

I dont think so, because it takes address from the system, u can change the address by sudo "ifconfig eth0 192.168.1.2" (but i dont know its consequences), still i will try to find answer of your question. :) – Gopak

Answer 2

A better approach will be:

 ifconfig eth0 | awk '/netmask/{split($4,a,":"); print a[1]}'

You can substitute the eth0 with any other interface you want

Slalom

this command didn't work for me. there was no output. I'm using ubuntu 18 – Lublin

fixed it. can you try? – Slalom

this gives the netmask not subnet – Etoile

Answer 3

INTERFACE=$(ip -o -f inet route |grep -e "^default" |awk '{print $5}')

echo $(ip -o -f inet addr show | grep "$INTERFACE" | awk '/scope global/ {print $4}')

Wyne

Answer 4

A simple way of doing it for me, was:

IP=$(ifconfig eth0 | grep -w inet | cut -d" " -f10) # device IP, e.g. 11.1.1.43
IP_RANGE=$(echo $IP | cut -d"." -f1-3).0/24 # subnet 11.1.1.0/24

Replace of course eth0 with the right interface diplayed by ifconfig.

Rann

Answer 5

That's how I get the IP and subnet mask with bash/awk:

IFCONFIG=$(ifconfig eth0)
IPETH=$(echo "$IFCONFIG" | grep 'inet addr:' | cut -d: -f2 | awk '{ print $1}')
MASK=$(echo "$IFCONFIG" | awk '/Mask/{split($4,a,":"); split(a[2],m,"."); h=m[1]*16777216+m[2]*65536+m[3]*256+m[4]; s=0; for(i=0; i < 32; i++) { s+=and(h,1); h/=2 } print s; }')
echo ${IPETH}/${MASK}

Depending on your version of ifconfig you must use /Mask/ or /netmask/ to get the subnet mask. I need this bit fiddling because I don't have ip on my system.

This gives for me e.g.

172.29.11.12/24

Plangent