Section: 5.1 Identification for routeing purposes
Content: MSCs, GSNs, location registers and CSSs are identified by international E.164 numbers and/or Signalling Point Codes ("entity number", i.e., "HLR number", "VLR number", "MSC number", "SGSN number", "GGSN number" and "CSS number") in each PLMN. MMEs that support "SMS in MME" are identified by international PSTN/ISDN numbers for SM Routing via an IWF (i.e. "MME number for MT SMS"). Additionally SGSNs and GGSNs are identified by GSN Addresses. These are the SGSN Address and the GGSN Address. A GSN Address shall be composed as shown in figure 9. Figure 9: Structure of GSN Address The GSN Address is composed of the following elements: 1) The Address Type, which is a fixed length code (of 2 bits) identifying the type of address that is used in the Address field. 2) The Address Length, which is a fixed length code (of 6 bits) identifying the length of the Address field. 3) The Address, which is a variable length field which contains either an IPv4 address or an IPv6 address. Address Type 0 and Address Length 4 are used when Address is an IPv4 address. Address Type 1 and Address Length 16 are used when Address is an IPv6 address. The IP v4 address structure is defined in RFC 791 [14]. The IP v6 address structure is defined in RFC 2373 [15].