CSC 600 Internetworking

withTCP/IP

Unit 4: Internet Addresses

(Ch. 4, 5, 6, 10)

Dr. Cheer-Sun Yang

Spring 2001

Internetworking Terms (1)• Communications Network

– Facility that provides data transfer service

• An internet– Collection of communications networks interconnected by

bridges and/or routers

• The Internet - note upper case I– The global collection of thousands of individual machines and

networks

• Intranet– Corporate internet operating within the organization– Uses Internet (TCP/IP and http)technology to deliver documents

and resources

Internetworking Terms (2)

• End System (ES)– Device attached to one of the networks of an

internet– Supports end-user applications or services

• Intermediate System (IS)– Device used to connect two networks– Permits communication between end systems

attached to different networks

Internetworking Terms (3)• Bridge

– IS used to connect two LANs using similar LAN protocols– Address filter passing on packets to the required network only– OSI layer 2 (Data Link)

• Router– Connects two (possibly dissimilar) networks– Uses internet protocol present in each router and end system– OSI Layer 3 (Network)

Internetworking Protocols

IP Operation

Three Levels of Addresses• Ethernet address (p. 30): appeared in a LLC frame;

used by firmware on the NIC to identify a host machine. An Ethernet frame:

• IP Address: appears in an IP datagram(p. 98); used by IP level software to identify source or destination.

• TCP port: appeared in a TCP segment (P. 221); used for identifying an application, i.e., TELNET, FTP, etc.

Preamble Dest. Source Type Data CRC

8 6 6 2 46-1500 4

IP ADDRESS FORMAT

IP Addresses - Class A

• 32 bit global internet address• Network part and host part• Class A

– Start with binary 0– All 0 reserved– 01111111 (127) reserved for loopback– Range 1.x.x.x to 126.x.x.x– All allocated

IP Addresses - Class B

• Starts with 10• Range 128.x.x.x to 191.x.x.x• Used for medium size networks that have

between 28 and 216 hosts.• Second octet also included in network address• 214 = 16,384 class B addresses• All allocated

IP Addresses - Class C

• Starts with 110• Range 192.x.x.x to 223.x.x.x• Used for networks that have less than 28 hosts.• Second and third octet also part of network

address• 221 = 2,097,152 addresses• Nearly all allocated(See IPv6)

Addresses Specify Network Connections

• To simplify the discussion, we said that an internet address identifies a host. But this is not strictly correct.

• If a host is moved from one LAN to another, the IP address must be changed.

• So an IP address does not identify a host.

Addresses Specify Network Connections

• Rather, an IP address identifies a network connection.

• Another example is a multi-homed host. For each connection, the host must be assigned an IP address.

Network and Directed Broadcast Addresses

• Why is network information encoded in an IP address?

Network and Directed Broadcast Addresses

• Why is network information encoded in an IP address?– Routing can become more efficient– IP addresses can refer to networks as well as hosts.– If the hostid consists of all 0s, the IP address refers

to the network.– If the hostid consists of all 1s, the IP address refers

to broadcasting to all nodes on the network.(directed broadcast)

Limited Broadcast Addresses

• A directed broadcast address can be interpreted unambiguously at any point in the Internet because it uniquely identifies the target network in addition to specifying broadcast on that network.

• Disadvantage: it requires knowledge of the network address.

• Another form: limited broadcast or local broadcast address

Limited Broadcast Addresses

• A limited broadcast address contains thirty-two 1s.

• A host may use the limited broadcast address as part of a start-up procedure before it learns its IP address.

Interpreting Zero to Mean “This”

• An IP address 0.0.0.0 means “this.”

• An IP address with netid 0 refers to “this network.”

• Using netid 0 is especially important in those cases where a host wants to communicate over a network but does not yet know the network IP address. All other hosts interprete the address as meaning “this” network.

Loopback

• Any IP address starting with 127 is considered a loopback address.

• It means that a frame will not go out of the machine. Instead, it will be passed to the receiving module of the IP layer software as if the frame is received from outside.

• It is used for testing IP configuration.

• For example: ping 127.0.0.1

Masking

• Masking is a process that extracts the address of the physical network from an IP address.

• For example: the mask for an IP address 128.175.1.1 to extract the netid is 255.255.0.0. (Why?)

Class D & Class E

• Class D – Multicast

• Class E – reserved for future use.

Dotted Decimal Notation

SOME SPECIAL FORMS OF ADDRESSES

EXAMPLE

Class B

Class C

Class A

128.10.2.70

192.5.48.7192.5.48.6

10.0.0.37

EXAMPLEI am “single”homed!

multi-homed

I am a router!So am I!

ARP

• Mapping internet addresses to physical addresses - the address resulation problem

An Example

• Whenever we type a command “ftp 144.26.77.43” (an Ethernet host) on coyote.cs.wcupa.edu (144.26.28.51).

• The sending host must convert the 32-bit IP address into the physical address for 144.26.77.43.

• Check this with TCPDUMP -e “arp” on taz

Address Resolution

• Some keeps tables in each machine that contains pairs of high-level and physical addresses.

• Others encode hardware addresses in high-level addresses.

• TCP/IP uses two techniques– direct mapping– dynamic mapping

Direct Mapping• Consider a token ring, e.g., proNET, that uses small

integers for physical addresses and allows users to choose a hardware address when installing an interface board in computer.

• One has the freedom to choose both the IP and the physical addresses.

• We can choose the physical address as part of the IP address.

• For example the physical address 3 has IP address 192.5.48.3

Direct Mapping

• Efficient

• But may not work for all networks, e.g., Ethernet

• Ethernet interface card is assigned a physical address of 48 bits

• But an IP address has 32 bits

• A machine’s physical address can change

• A machine can have two or more addresses.

Dynamic Mapping

• The designer of TCP/IP devised a low-level protocol to bind IP addresses to physical addresses dynamically.

• Address Resolution Protocol (ARP)

• When a host A wants to resolve IP address IB, it broadcasts the IP address. Only host B would reply with the physical address to A.

A Question

• Isn’t it silly to broadcast a request first to obtain the IP address prior to sending a frame? Why not broadcasting the frame with a physical address of the destination?

• Isn’t it the case anyway when sending out a frame?

Why?• It is costly to broadcast twice for every send

operation.

• After sending a request and reply once for each machine, it is cheaper to keep the mapping in a local cache.

• It is cheaper to look it up locally first prior to sending out a frame.

• All machine can use IP address to identify a host - one level of abstraction

ARP Cache Maintenance

• Cache opens up another can of worms!

• A host may change it’s IP address or hardware address without notifying everyone.

• A timer is associated with each entry in the cache. When the timer expires, the entry is deleted-trade efficiency with robustness.

• Command: /sbin/arp -a

ARP Refinements• When A broadcasts a request, A also includes it’s

IP address and physical address in the ARP request.

• Every host will save a copy of the mapping about A.

• When the response is broadcast, eveery host also keeps a copy of the mapping about B.

• When a machine replaces it’s physical address, notify others at boot time.

ARP Encapsulations and Identification

ARP Message Format

RARP

• Determining an internet addresses at startup.

• Usually an IP address is kept on a local disk. If a machine doesn’t have a disk, how can the machine determine its IP address?

• When bootstrap code is executed, it uses the network to contact a server and obtain the computer’s IP address.

A B C D

A sends B, C, D a broadcast request.

B, C, D authorized to supply the RARP service (C and D) reply directly to A.

Chapter 10

ClassLess and Subnet Address Extensions

ROADS Problem

• Including all 0’s and all 1’s, the hostid in – class A: 216 <= hostid <= 224 - 1– class B: 28 <= hostid <= 216 -1 – class C: 0-255

• It is apparent that requiring a unique prefix for each physical network would exhaust the address space quickly.

• Running Out of Address Space (ROADS).

Solutions

• The address space of IP addressing scheme becomes exhausted.

• Several ways to minimize the number of addresses used:– Transparent routers– proxy ARP– subnetting

Transparent Routers

The Local Area Network does not have its IP prefix; hosts attached to it areassigned addresses as if they are connected directly to the WAN.

Transparent Routers

• A host does not have to tell the router where a packet should be sent to. The router knows where to send using default information and history.

Transparent Routers

• Transparent routers can divide the IP address into multiple parts and encode information in unused parts. For example, the ARPANET was assigned the address 10.0.0.0 and each packet switch node (PSN) on the ARPANET had a unique integer address.

Transparent Routers

• Internally, the ARPANET treated any 4-octet IP address of the form 10.p.u.i as four separate octets that specify a network (10), a specific port on the destination PSN(p), and a destination PSN (i). Octet u remains uninterpreted.

• Thus, the addresses 10.2.5.37 and 10.2.9.37 both refer to host 2 on node 37.

Proxy ARP

Subnets and Subnet Masks• Allow arbitrary complexity of internetworked LANs

within organization• Insulate overall internet from growth of network numbers

and routing complexity• Site looks to rest of internet like single network• Each LAN assigned subnet number• Host portion of address partitioned into subnet number and

host number• Local routers route within subnetted network• Subnet mask indicates which bits are subnet number and

which are host number

Subnets and Subnet Masks• Example 1: An organization with a class A

address needs at least 1000 subnetworks. Find the subnet mask and configuration of each subnetwork.

• Without subnetting: Mask = 255.0.0.0• With subnetting: Mask = ?

Subnets and Subnet Masks• Example 1: An organization with a class A address

needs at least 1000 subnetworks. Find the subnet mask and configuration of each subnetwork.

• Without subnetting: Mask = 255.0.0.0• With subnetting: Mask = 255.255.192.0

– 11111111-11111111-11000000-00000000

• Divide the original space range into 1024 ranges, with two ranges to be reserved as special addresses. There are 16384 addresses in each range. There are 16382 (why?) possible computers.

Subnets and Subnet Masks• Example 2: An organization with a class B address

needs at least 12 subnetworks. Find the subnet mask and the configuration.

• Subnet mask = 255.255.240.0– 11111111 11111111 11110000 00000000

• The original hostid addresses have been divided into 16 ranges, with two ranges reserved as special addresses.

• There are 212(=4096) addresses in each range.• Only 4094 computers can be attached to each

subnetwork.

Variable-Length Subnetting• The Internet allows a site to use a variable-length

subnetting.• As an example, a site has the following number of

hosts in 5 subnetworks with class C addresses: 60, 60, 60, 30, 30.

• What subnet mask should it be?– Using 2 bits for the subnet id, we can have only 4

subnets– Using 3 bits for the subnet id, we can have only 32

hostid addresses

Variable-Length Subnetting• What subnet mask should it be?

– Using 2 bits for the subnet id, we can have only 4 subnets

– Using 3 bits for the subnet id, we can have only 32 hostid addresses

• A router maintains two subnet masks:

Variable-Length Subnetting• A router maintains two subnet masks:

62 hosts

62 hosts

30 hosts

Router

255.255.255.192

255.255.255.224

62 hosts

30 hosts

Subnetting

CIDR Address Blocks and Bit Masks

• Classless addressing is another name of supernetting.

Supernetting• The space of a class C address, with a maximum number of

254 host addresses, may not satisfy the need of an organization.

• Supernetting: apply for a group of class C addresses instead of 1.

• For example, an organization that needs 1000 addresses can be granted 4 class C addresses:– X.Y.32.1-X.Y.32.254– X.Y.33.1-X.Y.33.254– X.Y.34.1-X.Y.34.254– X.Y.35.1-X.Y.35.254

• Network mask = 255.255.252.0 for netid

Supernetting• For example, an organization that needs 1000

addresses can be granted 4 class C addresses:– X.Y.32.1-X.Y.32.254– X.Y.33.1-X.Y.33.254– X.Y.34.1-X.Y.34.254– X.Y.35.1-X.Y.35.254

• Network mask = 255.255.252.0 for netid• X.Y.33.4 belongs to the supernet X.Y.33 • X.Y.36.4 doesn’t.• We will come back to the subnetting and supernetting again when we

discuss Classless InterDomain Routing (CIDR)

CIDR Address Blocks and Bit Masks

• CIDR requires both an address and a mask, a shorthand notation was devised.

• The block of addresses in Fig. 10.10 can be expressed as 128.211.168.0/21, where /21 denotes 21 bits in a mask.

• The /8, /16, and /24 prefixes correspond to traditional class A, B, and C divisions.

A Classless Addressing Example

• If an ISP is assigned 128.211.0.0/16.

• The ISP can assign 128.211.0.0/16 to a customer with 2048 address in the /21 range that Fig. 10.10 specifies.

• The ISP can also assign 128.211.176.212/29 to cover the address range that Fig. 10.12 specifies.