Classful Internet Addresses Chapter 4 Universal Identifiers Designers

Classful Internet Addresses Chapter 4

Universal Identifiers • Designers of TCP/IP determined that each host on the internet would have a 32 -bit identifier called an IP address – The address of the network is included in the address (the prefix) of the host on the network – The IP addresses of hosts on a network share a common prefix • Each address is a pair (netid, hostid), where netid identifies the network, and hostid identifies the host on that network

The Original Addressing Scheme • The original addressing scheme was called classful • See Figure 4. 1 – Class A addresses - used for few networks with many hosts – Class B addresses - used for intermediate size networks – Class C addresses - used for networks with < 28 hosts – Class D addresses - internet multicasting – Class E addresses - future

Addresses Specify Network Connections • In general, an internet identifies a host • If a router connects to two networks, then it requires two IP addresses, one for each network • Conventional computers that attach to more than one network are called multi-homed hosts

Network Addresses • A hostid of 0 (all zeroes) is not assigned to an individual host, it indicates the network itself • A hostid of all ones is a broadcast address and refers to all hosts on the network (ones means all) • When a packet is broadcast: – a receiving router looks at the netid portion of the address to pass it on to that network – when the packet arrives at the destination network, that router looks at the hostid; if all ones, it broadcasts

Limited Broadcast • As part of a startup procedure, a host may use a limited broadcast address (32 ones) before it knows its IP address or the prefix for the local network • Generally, TCP/IP restricts broadcasting to the smallest set of machines

Zeros and Ones • A field consisting of ones can be interpreted to mean “all” • A field consisting of zeros can be interpreted to mean “this” – hostid = 0 is this host – netid = 0 is this network

Addressing Extensions • To conserve network prefixes, subnet addressing allows multiple physical networks to share prefixes • In the 1990’s, it was determined that the division between prefix and suffix could occur at any point; this was called classless addressing, or supernetting • For right now, we just need to know that the original scheme is no longer the most widely used

Weaknesses in Internet Addressing • Addresses refer to network connections, not the computer itself – If a host is moved from one network to another, its IP @ must change – Problem with mobile computers; when reconnected at a remote site, it needs an IP @ at this new network? • Class C networks can have no more than 255 hosts • Which @ do we use if a host is multi-homed? • See Figure 4. 2

Dotted Decimal Notation • IP addresses are written as four decimal digits, separated by decimal points • Each decimal corresponds to one octet in the 32 bit IP address • The 32 -bit internet address 10000001110 00001001 00011101 is written as 129. 13. 9. 29 • See relationship of IP @ and dotted decimals in Figure 4. 3

Loopback Address • 127. 0. 0. 0 is used for testing software • The protocol software on the computer interprets this locally and sends no traffic across the network • See summary of special addresses in Figure 4. 4

Internet Addressing Authority • ICANN (Internet Corporation for Assigned Names and Numbers) sets policy and assigns values for names and addresses – Previously was IANA (Internet Assigned Number Authority) until 1998 and was handled by Jon Postel • This central authority assigns the network portion of the address • UHCL has 16 subnets 160 - 175. For example: – 129. 7. 160. 1 – 129. 7. 173. 15

Corporate Groups • IBM has network address 9. 0. 0. 0 • AT&T has 12. 0. 0. 0 • If a corporation has no connections to the Internet, it may use these addresses. Would it be wise? • Why would someone have no connection to the Internet? • The IETF has reserved address prefixes and recommends using them on private internets

Example from Purdue • Mid 1980’s • IP addresses are assigned to each network connection • Merlin has 2 addresses • IP addresses for the routers (Glatisant and Taliesyn) do not follow the convention

Network Byte Order • Not all architectures store 32 -bit integers the same way – Little Endian - low memory address holds low-order byte of the integer – Big Endian - low memory holds high-order byte – How did you do it in Assembly? • In TCP/IP, hosts and routers convert binary to standard byte order before sending a packet, and to host-specific order when receiving • TCP/IP uses Big Endian

Summary • TCP/IP uses 32 -bit binary addresses (called IP addresses) for machine identifiers – address consists of a network id and a host id • The classful scheme worked for a while, but had to be extended • IP addresses refer to network connections • A standard byte-ordering scheme is necessary

For Next Time • Read Chapter 5