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 < 2 8 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 must change –Problem with mobile computers; when reconnected at a remote site, it needs an at this new network? Class C networks can have no more than 255 hosts 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 is written as See relationship of and dotted decimals in Figure 4.3

Loopback Address 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 For example: – –

Corporate Groups IBM has network address AT&T has 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