IP addresses have hierarchy (network & subnet) Internet names (FQDNs) also have hierarchy. and of course there can be sub-sub-!!

Transcription

1 The Domain Hierarchy IP addresses have hierarchy (network & subnet) Internet names (FQDNs) also have hierarchy the general form for a fully qualified name is and of course there can be sub-sub-!! -sub-domains next page shows logical picture of the domain hierarchy

2 www uiowa cs math weblog edu iastate utexas yahoo com amazon jp ac co dendai hosts shown are""" # $%&' " ()*+,",- '/0# $%&' " ()*+ 2

3 to support the domain hierarchy, there is a distributed database several types of records in this DNS database (table in Chapter 8; details in Appendix C) type A record has hostname and IP address fields type NS record domain s nameserver type CNAME record define alias hostname many (hundreds of thousands) name servers in Internet each name server has only a fraction of the records (basically just the records for its domain) all the name servers are in a hierarchy 3

4 edu NS uiowa NS iastate NS utexas NS 2 cs,math,stat3 NS yahoo NS root NS com NS amazon NS jp NS ac NS co NS dendai NS in reality, less correspondence between servers and domains 4

5 important! name servers are not low-level protocols like ARP, but are applications application, transport, network, and physical layers do not need domain or host names IP numbers are enough for networking (but not so pleasant) basic nameserver idea when host needs FQDN4 IP-addr translation, it asks the local name server (TCP to well-known port at nameserver host) how does it know IP address of local name server? 2 if local name server has answer (in local database), send response 3 if not in local database, check for desired name in cache (similar to arp strategy) 4 failing above, forward query to parent nameserver so each NS needs pointer record in database to parent IP-address 5 continued 6 at higher level in hierarchy, NS forwards query to a child NS having more specific knowledge about the domain in question and so on 5

6 problem: how to build a plug-and-play host that can use the Internet without having a built-in IP address? solution idea: have an administrator host on LAN that gives or loans an IP number to such a plug-and-play host solution is two step solution: new host on LAN broadcasts request to get an IP address 2 a server on LAN sends requestor a message with IP address this type of solution has been implemented in three different protocols, RARP, BOOTP, and DHCP RARP solution mainly using physical layer client broadcasts RARP query frame on Ethernet (recall: each frame has source Ethernet address) server receives broadcast; looks up source Ethernet address (lookup in table on disk) finds entry with matching Ethernet address, replies with corresponding IP number disadvantage: relies on table with known Ethernet addresses; table has to be maintained by somebody 6

7 BOOTP protocol first, recall some background about special IP addresses a5 b5 c5 d with zero bits in host part of address 6 address a5 b5 c5 d is not a host, but is the network address a5 b5 c5 d with one bits in host part of address 6 address a5 b5 c5 d is not a host, but is the broadcast address address also is LAN broadcast address UDP can sent a packet to all hosts on LAN by using broadcast address how BOOTP works client first uses UDP broadcast to to send BOOTREQUEST packet server broadcasts BOOTREPLY packet with assigned IP number server still uses file with hardware (Ethernet) address and IP address table entries, manually entered by somebody why does server use broadcast rather than unicast? 7

8 Dynamic Host Configuration Protocol (DHCP) generalize BOOTP to flexible, dynamic service DHCP features allow a pool of IP addresses any requesting client can get IP address from pool assigned IP addresses from pool are borrowed client can request extension when borrow-time expires still allows manually assigned IP addresses (useful for printers and file servers) 8