4 knows the IP address of the name server main- tained by the network manager of company B. To ensure that domain name servers can reach each other, all name servers in the tree must know the (Root) (France) (Finland) (Sweden) CompanyA CompanyB CompanyC Host A Development Marketing Production Host A Host B hosta.development.companyb.fr hosta.production.companyb.fr Figure 6.54 Hierarchy of the DNS. Introduction to Telecommunications Network Engineering address of at least one root server. Then all servers are accessible at least via the root server. The IP address is enough for DNS message exchange because all DNS servers use UDP and port 53 for communication. 6.6.10.6 SNMP SNMP is a protocol for the transfer of network management messages between network elements, such as routers and switches, and network man- agement center computers. SNMP defines messages to be exchanged; it has no functionality for actual management actions. Most routers, LAN switches, and other LAN devices support SNMP, and many network management software packages are available for network control and monitoring with the help of SNMP. 6.6.10.7 DHCP Each computer attached to the Internet needs to know its IP address before it can send or receive datagrams [4]. In addition the computer needs other infor- mation such as address of a default router, the subnet mask to use, and the address of the name server. DHCP can provide all the needed information. DHCP uses IP and UDP protocols, and the payload of a UDP datagram contains a DHCP message. With the help of DHCP IP addresses need not be permanently assigned. Each time when the host joins the network, or is pow- ered up, it requests an IP address and other needed information from the DHCP server. To access the server whose address it does not know, the client uses IP broadcast address 255.255.255.255 (see Figure 6.39). The client's hardware or MAC address is attached to the DHCP message. Also the server has to use an IP broadcast address in the response but by knowing the hard- ware address the 