The APN Network Identifier shall contain at least one label and shall have, after encoding as defined in clause 9.1 above, a maximum length of 63 octets. An APN Network Identifier shall not start with any of the strings "rac", "lac", "sgsn" or "rnc", and it shall not end in ".gprs", i.e. the last label of the APN Network Identifier shall not be "gprs". Further, it shall not take the value "*". In order to guarantee uniqueness of APN Network Identifiers within or between GPRS/EPS PLMN, an APN Network Identifier containing more than one label shall correspond to an Internet domain name. This name should only be allocated by the PLMN if that PLMN belongs to an organisation which has officially reserved this name in the Internet domain. Other types of APN Network Identifiers are not guaranteed to be unique within or between GPRS/EPS PLMNs. An APN Network Identifier may be used to access a service associated with a GGSN/PGW. This may be achieved by defining:

• an APN which corresponds to a FQDN of a GGSN/PGW, and which is locally interpreted by the GGSN/PGW as a request for a specific service, or
• an APN Network Identifier consisting of 3 or more labels and starting with a Reserved Service Label, or an APN Network Identifier consisting of a Reserved Service Label alone, which indicates a GGSN/PGW by the nature of the requested service. Reserved Service Labels and the corresponding services they stand for shall be agreed between operators who have GPRS/EPS roaming agreements.

The APN Operator Identifier is composed of three labels. The last label (or domain) shall be "gprs". The first and second labels together shall uniquely identify the GPRS/EPS PLMN. For each operator, there is a default APN Operator Identifier (i.e. domain name). This default APN Operator Identifier is derived from the IMSI as follows: where: This default APN Operator Identifier is used for home routed inter-PLMN roaming situations when attempting to translate an APN consisting only of a Network Identifier into the IP address of the GGSN/PGW in the HPLMN. The PLMN may provide DNS translations for other, more human-readable, APN Operator Identifiers in addition to the default Operator Identifier described above. Alternatively, in the roaming case if the GGSN/PGW from the VPLMN is to be selected, the APN Operator Identifier for the UE is constructed from the serving network PLMN ID. In this case, the APN-OI replacement field, if received, shall be ignored. In order to guarantee inter-PLMN DNS translation, the <MNC> and <MCC> coding used in the "mnc<MNC>.mcc<MCC>.gprs" format of the APN OI shall be:

• <MNC> = 3 digits
• <MCC> = 3 digits
• If there are only 2 significant digits in the MNC, one "0" digit is inserted at the left side to fill the 3 digits coding of MNC in the APN OI.

As an example, the APN OI for MCC 345 and MNC 12 will be coded in the DNS as "mnc012.mcc345.gprs". The APN-OI replacement is used for selecting the GGSN/PGW for non-roaming and home routed scenarios. The format of the domain name used in the APN-OI replacement field (as defined in TS 23.060 and TS 23.401) is the same as the default APN-OI as defined above except that it may be preceded by one or more labels each separated by a dot. It is up to the operators to determine what labels shall precede the "mnc<MNC>.mcc<MCC>.gprs" trailing labels (see clause 5.1.1.1 of TS 29.303 and also clause 13.1 of TS 23.060). The APN constructed using the APN-OI replacement field is only used for DNS translation. The APN when being sent to other network entities over GTP interfaces shall follow the rules as specified in TS 23.060 and TS 23.401.

The wild card APN is coded as an APN with "*" as its single label, (i.e. a length octet with value one, followed by the ASCII code for the asterisk).