olemus
(1) IPv4 aadressiprefiks
= address prefix in IPv4
http://www.tcpipguide.com/free/t_IPBasicAddressStructureandMainComponentsNetworkIDa.htm

http://www.freesoft.org/CIE/Course/Subnet/5.htm

(2) välist võrku määratlev osa pääsupunktinimes
=the part of the access point name that defines the external network
http://telecommguide.blogspot.com.ee/2014/09/apn-access-point-name.html

https://en.wikipedia.org/wiki/Access_Point_Name