LTE APNMobile Devices Making Data Connections must be Configured with an LTE APN
Within the Long Term Evolution (LTE) network infrastructure are many LTE identifiers that help move things along. One of these identifiers is LTE APN. The help that APN brings is specifically important, as it is responsible for connectivity requests, which basically means it’s the one left asking for permission when it comes time to connect to another network. This is a constant and necessary task for the LTE network. Access Point Name (APN) is an identifier that lives in the LTE core network, otherwise known as the Evolved Packet Core (EPC). In that home, the APN comes into play inside the Home Subscriber Server (HSS) node of the core network.
Where You’ll See LTE APN
The HSS contains users’ SAE subscription data such as the EPS-subscribed Quality of Service (QoS) profile and any access restrictions for roaming. HSS also contains information about the (Pocket Data Networks) PDNs to which the user can connect, as reported by Alcatel-Lucent. “This could be in the form of an access point name (APN) (which is a label according to DNS naming conventions describing the access point to the PDN) or a PDN address (indicating subscribed IP address(es)). In addition the HSS holds dynamic information such as the identity of the MME to which the user is currently attached or registered. The HSS may also integrate the authentication center (AUC), which generates the vectors for authentication and security keys.” 
What LTE APN Does
To break it down, the APN identifies a Gateway GPRS Support Node (GGSN) or Packet Data Network GateWay (P-GW). It includes an APN network identifier which defines the Packet Data Network (PDN) to which the UE requests connectivity, and may also include an APN operator identifier which defines in which Public Land Mobile Network (PLMN) the P-GW or GGSN is located, according to LTE World.  To accomplish this, the APN structure is comprised into two parts: a network identifier and an operator identifier.
How to identify?
There are also steps for identifying a PDN IP network that the mobile data user wants to communicate with. The NMC Consulting Group notes that the PDN Identity (APN) is used to determine the P-GW and point of interconnection with a PDN. With APN as query parameter to the DNS procedures, the MME will receive a list of candidate P-GWs, and then a P-GW is selected by MME with policy. 
The board who sets the standards for 3GPP, however, sees things a little differently. According to the new standards puts in place, the UE shall not include APN and PCO in the PDN connectivity request when the same is sent along with attach request. 3GPP has said that the UE shall send the PDN connectivity request with a flag “ESM Information transfer” on and no APN or PCO shall be included. Once the MME receives the Attach Request+PDN connectivity request, it can move ahead and accept the attach but it still cannot establish the EPS bearers just yet.
Next, MME goes ahead with establishing security context. After the security context is established MME will send a NAS message “ESM Information Request” asking UE for APN and PCO. Then, the UE will send an “ESM Information Response” with APN and PCO, encrypted. Finally, once MME receives this response it will go ahed with establishing the EPS bearers. If the response doesn’t include APN then default APN shall be used by MME.
 The LTE Network Architecture, Alcatel-Lucent, page 3
 APN, LTE World, http://lteworld.org/category/glossary/apn
 LTE Identifiers, NMC Consulting Group, http://www.nmcgroups.com/files/download/NMC.LTE%20Identifiers.v1.0.pdf