UNIX for Advanced & Expert Users

sed '${/^ACL localnet/{s/.*/&\nMy Network 192.168.1.1/}}' infile

lem@biggy:/tmp$ line=$(sed -n '/^ACL localnet/ =' file |tail -n1)
lem@biggy:/tmp$ (( ${line:-0} == 0 )) || sed  "$line a\
My Network 192.168.1.1" <file >newfile

 function print_cache( rec ) { if( ! buffer ) return; close( cache ); while( (getline rec <cache ) > 0 ) print rec; close( cache ); } /^ACL localnet/ { print_cache(); print; buffer = 1; next; } { if( buffer ) print >cache; else print; next; } END { if( buffer ) print "My Network 192.168.1.1 "; print_cache( ); system( "rm " cache ); } ' input >output

sed "$(awk '/^ACL localnet/ {n=NR} END{print  n}' infile)  a \My Network 192.168.1.1" infile
|                                                            ^---  edited after Don Cragun's below post

sed: 1: "14  a My Network 192.16 ...": command a expects \ followed by text