CSC 600 Internetworking with TCP/IP

1. CSC 600InternetworkingwithTCP/IP Unit 4: Internet Addresses (Ch. 4, 5, 6, 10) Dr. Cheer-Sun Yang Spring 2001

2. 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

3. 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

4. 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)

5. Internetworking Protocols

6. IP Operation

7. 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.

8. IP ADDRESS FORMAT

9. 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

10. 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

11. 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)

12. 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.

13. 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.

14. Network and Directed Broadcast Addresses • Why is network information encoded in an IP address?

15. 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)

16. 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

17. 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.

18. 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.

19. 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

20. 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?)

21. Class D & Class E • Class D – Multicast • Class E – reserved for future use.

22. Dotted Decimal Notation

23. SOME SPECIAL FORMS OF ADDRESSES

24. EXAMPLE Class B Class A 10.0.0.37 128.10.2.70 192.5.48.6 192.5.48.7 Class C

25. EXAMPLE I am “single”homed! multi-homed I am a router! So am I!

26. ARP • Mapping internet addresses to physical addresses - the address resulation problem

27. 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

28. 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

29. 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

30. 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.

31. 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.

33. 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?

34. 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

35. 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

36. 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.

37. ARP Encapsulations and Identification

38. ARP Message Format

39. 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.

40. 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.

41. Chapter 10 ClassLess and Subnet Address Extensions

42. 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).

43. Solutions • The address space of IP addressing scheme becomes exhausted. • Several ways to minimize the number of addresses used: • Transparent routers • proxy ARP • subnetting

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

45. 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.

46. 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.

47. 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.

48. Proxy ARP

49. 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

50. 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 = ?