Assumption : I installed ns2 under the folder /opt/ The path in my system is /opt/ns-allinone2.33/ns2.33/ U nder the above said folder there are two Makefiles ( Makefile and Makefile.in ) Makefile.in is the input file (any addition or correction will be done in this file only). during compilation this file will be copied to Makefile . So dont try to modify Makefile . Once the changes are made to the Makefile.in, execute the command ./configure at the above said folder. (/opt/ns-allinone2.33/ns2.33# ./configure) Then execute make, if there are any errors during make, correct it . For Sample Examples of recompilation and how to edit the Makefile.in, comment me, so that i will put it in the next post.