entifier (Ci), called destination local reference). Although not shown by the model, there exists management types of layers as well that run on TCP or UDP, e.g., BGP (an internetwork route management protocol), DNS (the domain name system, see Sect. 3.4.5.3) and SNMP (the Internet management system). 3 Node Structures 3.4.5 Global Routes 3.4.5.1 Introduction Switched and socket routes have the disadvantage that hosted nodes must refer a route with a network address of a hosting node, which they cannot control and which cannot be easily changed, since it is used for routing in the hosting network. Hosted nodes may be reallocated to other switching nodes for many reasons, and must then normally be addressed with a new network address. Such changes will affect other nodes of the same hosted network, by requiring modification of their T-RE tables. An alternative method of handling this problem would be to modify the T-ROs tables of switching nodes, so that hosted node can use the same network address even when reallocated to another hosting node. This is normally not realis- tic for large switching networks. This problem is more serious in open networks such as the Internet, but similar needs have been felt in SS7, as well as in public networks such as PSTN/ISDN. The solution is to allow hosted networks to define and use their own numbering system, separated from the numbering system of the hosting network. The princi- ple is depicted in Fig. 3.30. T-GT(n21): n32->{n22, destAPi}, where n32 is a GT RE(n21); n22-NA(n22) T-RE(n21): n22-NA(n22) Global routes Switched NA ((22) routes NA(n22) routes Fig. 3.30 Realizing global routes. GT=global title; T-GT=GT {REi,APi} To the left is a switching logical node n11 that applies the layer APi method. n11 hosts a number of logical nodes, e.g., n21, that rely on switched routes that are addressed by NAs. These NAs are defined by the numbering system of the nw1 switching network. To the right we have transformed the logical network nw2 to a hosting network that applies the layer-