The best way is of course to set nodename in command line through "-sname node" or "-name node@host". But it is possible to use `net_kernel' module instead. It is described at http://www.erlang.org/doc/man/net_kernel.html
$ erl
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.9.1 (abort with ^G)
1> node().
nonode@nohost
2> net_kernel:start([rumata, shortnames]).
{ok,<0.34.0>}
(rumata@rumata-osx)3> node().
'rumata@rumata-osx'
(rumata@rumata-osx)4> net_kernel:stop().
ok
5> node().
nonode@nohost
6> net_kernel:start(['rumata@myhost', longnames]).
{ok,<0.44.0>}
(rumata@myhost)7> node().
rumata@myhost