You can download ipk packages from Local Opkg Repositories as described above, or from Remote Repositories specified as supporting Opkg packages.
To specify that a Remote Repository supports Opkg packages, set its Package Type to Opkg when it is created.
You can either point the remote to a specific feed (location of a Packages file), i.e. http://downloads.openwrt.org/chaos_calmer/15.05/adm5120/rb1xx/packages/luci
Or you can specify some base level and point your client to the relevant feeds in it i.e. url is http://downloads.openwrt.org/chaos_calmer/15.05/
and your opkg.conf
file has the entry src adm5120/rb1xx/packages/luci
Note that the index files for remote Opkg repositories are stored and renewed according to the Retrieval Cache Period setting.