detail in Chapter 6. E-mail programs receive LDAP support to search e-mail addresses. We use directories of one sort or another every time we use the Internet or our own intranets. LDAP is an extensible, vendor-independent, network protocol standard - it supports hardware and software. An LDAP-based directory supports any type of data. Remote Authentication Dial-In User Service (RADIUS) RADIUS is an authentication and accounting system used by many Internet service providers. When you dial in to the ISP you must enter your username and password. This information is passed to a RADIUS server, which checks that the information is correct, and then authorizes access to the ISP system. Local DNS server Other DNS servers Local IP network (access) Internet Client Server www.ericsson.se HTTP/DNS Ethernet -X.25 TCP/UDP Ethernet Eth.|X.25 Figure 8.36 A communication with a DNS server. The DNS is hierarchical with local servers at the bottom and root servers (e.g. com) at the top Understanding Changing Telecommunications - Building a Successful Telecom Business The use of RADIUS for address allocation is optional. An example is found in Figure 8.41 at the end of this chapter. A RADIUS server could be configured to always allocate the same IP address to a specific subscriber. Static IP address allocation enables the operator to offer subscribers the ability to use IP addresses that have been provided by the subscriber, which can assist with their security issues. Static IP address allocation means that subscribers can provide their own IP addresses. Dynamic Host Configuration Protocol (DHCP) The main function here is to administer the allocation of addresses. This may be based on authentication of the entity requesting the address. This function may also apply some policy to determine the type of address allocated (private or globally routable) and the period for which the address is allocated. There are several protocol standards for IP address allocation, e.g. DHCP. DHCP enables individual computers on an IP network to ex