Chapter 4, slide: 1 CS 372 – introduction to computer networks* Friday July 23, 2010 Announcements: r Midterms are graded. r Lab 4 is posted. Acknowledgement: slides drawn heavily from Kurose & Ross * Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.

Internet addresses r A key aspect of a virtual network is a single, uniform address format r Can't use hardware addresses because different technologies have different address formats r Can't use addresses that are local to a network because multiple networks might use the same addresses internally r Address format must be independent of any particular hardware address format Chapter 4, slide: 2

IP address notation r IP address is just a 32-bit number  It’s the same internally, regardless of its external representation  In decimal form, the range is [0 … ]  Some are reserved  In hexadecimal form, the range is [ … FFFFFFFF] divides naturally into 4 2-hexdigit groups e.g.:80 FD 28 1C each group represents one byte (octet) Binary form is r For convenience, use dotted decimal notation  e.g.: (= 80.FD.28.1C)  range is [ … ] Chapter 4, slide: 3

4 IP Addressing: introduction r IP address: 32-bit identifier for  host,  router interface r interface: connection between host/router and physical link  multiple interfaces per router  one interface per host  one IP address per interface =

Chapter 4, slide: 5 Subnets r IP address:  subnet part (higher bits)  host part (lower bits) r What’s a subnet ?  device interfaces with same subnet part of IP address  can physically reach each other without intervening router network consisting of 3 subnets subnet subnet part host part /23

Chapter 4, slide: 6 Subnets / / /24 Recipe r To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. Subnet mask: /24

Chapter 4, slide: 7 Subnets How many?

Chapter 4, slide: 8 IP addresses: how to get one? Q: How does host get IP address? r hard-coded by system admin in a file r DHCP: Dynamic Host Configuration Protocol:  dynamically get IP address from as server when joining the network  IP address can be reused by other hosts if released  Can renew IP addresses if stayed connected

Chapter 4, slide: 9 DHCP client-server scenario A B E DHCP server arriving DHCP client needs address in this network

Chapter 4, slide: 10 IP addresses: how to get one? Q: How does network get subnet part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block /20 Organization /23 Organization /23 Organization /23... ….. …. …. Organization /23

Chapter 4, slide: 11 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers  allocates addresses  manages DNS  assigns domain names, resolves disputes

IP address format r Each IP address is divided into a prefix and a suffix r Prefix identifies the network and the type of network to which a host computer is attached r Suffix identifies a host computer within that network r Usually includes indicator for number of bits used for prefix r Address format enables efficient routing Chapter 4, slide: 12

IP address hierarchy r Every network in a TCP/IP internet is assigned a unique network number r Each host on a specific network is assigned a host number or host address that is unique within that network r Host's IP address is the combination of the network number (prefix) and host address (suffix) Chapter 4, slide: 13

IP address assignment r Network numbers (prefixes) are unique r Host addresses (suffixes) may be duplicated on different networks r The combination of network number prefix and host address suffix is unique in the entire internet Chapter 4, slide: 14

IP address assignment r Assignment of network numbers must be coordinated globally r Assignment of host addresses can be managed locally Chapter 4, slide: 15

IP address design r IP-v4 designers chose 32-bit addresses r Allocate some bits for prefix, some for suffix  Large prefix, small suffix - many networks, few hosts per network  Small prefix, large suffix - few networks, many hosts per network r Because of the wide variety of technologies, need to allow for both large and small networks Chapter 4, slide: 16

IP addressing (two types): Classful addressing: A, B, C  A: /8  B: /16  C: /24 CIDR: Classless InterDomain Routing  network portion of address of arbitrary length  address format: a.b.c.d/x, where x is # bits in network portion of address network part host part /22 (only 2 8 networks, but 2 24 hosts per network) (2 16 networks, and 2 16 hosts per network) (2 24 networks, but only 2 8 hosts per network) Chapter 4, slide: 17

IP address classes: Classful addressing r Multiple address formats that allow both large and small prefixes r Each format is called an address class r The class of an address is identified by first four bits r The number of bits allocated for the prefix is determined by the class Chapter 4, slide: 18

IP address classes: Classful addressing Chapter 4, slide: 19

Chapter 4, slide: 20

IP address classes: Classful addressing r Class A, B and C are primary classes  Used for ordinary host addressing r Class D is used for multicast, a limited form of broadcast  Internet hosts join a multicast group  Packets are delivered to all members of group  Routers manage delivery of single packet from source to all members of multicast group  Used for multicast backbone r Class E is reserved Chapter 4, slide: 21

Determining IP class r Dotted decimal makes separating network address from host address easier r Look at first dotted decimal number, and use this table: Chapter 4, slide: 22

How many networks? r Classful scheme does not yield equal number of networks in each class r class A:  First bit must be 0  7 remaining bits identify Class A net  2 7 (= 128) possible class A nets Minus a few that are reserved r class B:  First 2 bits must be 10  14 remaining bits identify Class B net  2 14 (= 16384) possible class B nets Minus a few that are reserved r class C:  First 3 bits must be 110  21 remaining bits identify Class C net  2 21 (= ) possible class C nets Minus a few that are reserved Chapter 4, slide: 23

Networks and hosts in each class Chapter 4, slide: 24

What if the form doesn't fit? r Large organizations may not be able to get as many addresses in the Internet as they need r Example - UPS needs addresses for millions of computers r Example – School needs 6000 hosts  Too big for class C, too many wasted addresses for class B Chapter 4, slide: 25

Possible solutions r Classless addressing  allow division between prefix and suffix at any bit boundary r Sharing an IP address  Use one IP address for multiple hosts Chapter 4, slide: 26

Classless Addressing Example dotted decimal 80 C1 2F 19hexadecimal binary This can be “re-aligned” to use variable-size prefixes and suffixes. Example: Suppose we want a 22-bit prefix and a 10-bit suffix Chapter 4, slide: 27

Example: r r Logical AND with a "mask" using 22 bits for prefix (netmask), 10 bits for suffix: r … gives a prefix: r The complement mask (hostmask) r … gives a suffix: Chapter 4, slide: 28

Example: i.e., the “network number” is 80C12C00h, and the “host number” within the network is 319h r With 10 bits for suffix, 1024 host addresses are available within the subnet (but 2 of these are reserved) r Address still looks almost the same in dotted decimal  / 22  Additional information is provided so addressing can be handled by routers Chapter 4, slide: 29

CIDR r CIDR (Classless Inter-Domain Routing) address includes specification for number of bits to use for the netmask  Example:host address /22  What is the netmask?  What is the network address?  What is the hostmask?  What is the host number? = 319h = 793 (decimal) Chapter 4, slide: 30

CIDR r What is the netmask for /20 ?  r What is the netmask for /24 ?  r What is the netmask for /27 ?  r How many hosts can be supported in /28 ?  = 14 Chapter 4, slide: 31

Chapter 4, slide: 32 Example Subnet 1 Subnet 2 Subnet 3 r Three subnets r All interfaces in all these subnets are required to have prefix: /24 r Subnet 1 is required to support 125 interfaces r Subnet 2 & 3 are each required to support 60 interfaces r Question: Provide 3 network addresses in the form: a.b.c.d/x

IP address assignment r Select an address class for each network depending on expected number of hosts r Assign network numbers from appropriate classes r Assign host suffixes to form internet addresses for all hosts r The lowest host number (0) is not used because that complete address is the network address  e.g., r The highest host number (depends on network type) is not used because that complete address is the “broadcast address” for the network  e.g., Chapter 4, slide: 33

OSU IP addresses r Oregon State has a single Class B network: r All hosts at OSU have prefix r E.G.:  ns1.oregonstate.edu  ns2.oregonstate.edu r Suffix bytes are used to determine local network and host through subnetting r Individual host addresses assigned by system administrators  may be static or dynamic assignment Chapter 4, slide: 34

IP address allocation r Addresses in the Internet are not used efficiently  Less than 10% of possible addresses are actually assigned r Concerns about address space being exhausted r OSU is like most organizations, using 5,000- 6,000 out of possible 2 16 (= 65,536) available addresses Chapter 4, slide: 35

Routing table information r In the routing table  Destination stored as network address  Next hop stored as IP address of router r Address mask defines how many bits of address are in prefix  Prefix defines how much of address used to identify network  e.g., class B mask is In binary: Chapter 4, slide: 36

Example: Routing table for Chapter 4, slide: 37

Consider : Routing table for doesn’t care what does with the packets Chapter 4, slide: 38

Address masks r To identify destination network, apply address mask to destination address and compare to network address in routing table r Use Boolean and if ((Mask[i] & D) == Dest[i]) forward to NextHop[i] Chapter 4, slide: 39