Check the provided netmask is valid

[dglinder]

Basically look for 255.255.255.0 Three periods, four numbers, and digits on each end.

[dglinder]

From StackOverflow

https://stackoverflow.com/a/21746988/187426

NM="255.255.128.0"
echo ${NM} | grep -w -E -o '^(254|252|248|240|224|192|128).0.0.0|255.(254|252|248|240|224|192|128|0).0.0|255.255.(254|252|248|240|224|192|128|0).0|255.255.255.(254|252|248|240|224|192|128|0)' > /dev/null
if [ $? -eq 0 ]; then
echo "Valid netmask"
else
echo "Invalid netmask"
fi

[dglinder]

echo "IP address: ${network_ipaddr}/${network_netmask} via ${network_gateway}"
echo ${network_netmask} | grep -w -E -o '^(254|252|248|240|224|192|128).0.0.0|255.(254|252|248|240|224|192|128|0).0.0|255.255.(254|252|248|240|224|192|128|0).0|255.255.255.(254|252|248|240|224|192|128|0)' > /dev/null
if [ $? -ne 0 ]; then
echo "### ERROR >>>>>>-<<<<<< ERROR ###"
echo "--> Invalid netmask <--"
echo "### ERROR >>>>>>-<<<<<< ERROR ###"
fi