McGraw-Hill The McGraw-Hill Companies, Inc., 2000

Transcription

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3 Network Layer

4 Position of network layer

5 Figure 19.4 Network layer at the source

6 Figure 19.5 Network layer at a router

7 Figure 19.6 Network layer at the destination

8 19.2 Addressing Internet Address Classful Addressing Subnetting Network Address Translation

9 Note: An IP address is a 32-bit address. The IP addresses are unique and universal.

10 Figure 19.9 Dotted-decimal notation

11 Example 1 Change the following IP addresses from binary notation to dotteddecimal notation. a b Solution We replace each group of 8 bits with its equivalent decimal number (see Appendix B) and add dots for separation: a b

12 Example 2 Change the following IP addresses from dotted-decimal notation to binary notation. a b Solution We replace each decimal number with its binary equivalent a b

13 Note: In classful addressing, the address space is divided into five classes: A, B, C, D, and E.

14 Figure Finding the class in binary notation

15 Figure Finding the address class

16 Example 3 Find the class of each address: a b Solution See the procedure in Figure a. The first bit is 0; this is a class A address. b. The first 4 bits are 1s; this is a class E address.

17 Figure Finding the class in decimal notation

18 Example 4 Find the class of each address: a b c Solution a. The first byte is 227 (between 224 and 239); the class is D. b. The first byte is 252 (between 240 and 255); the class is E. c. The first byte is 134 (between 128 and 191); the class is B.

19 Figure Netid and hostid

20 Figure Blocks in class A

21 Note: Millions of class A addresses are wasted.

22 Figure Blocks in class B

23 Note: Many class B addresses are wasted.

24 Note: The number of addresses in class C is smaller than the needs of most organizations.

25 Figure Blocks in class C

26 Figure Network address

27 Note: In classful addressing, the network address is the one that is assigned to the organization.

28 Example 5 Given the address , find the network address. Solution The class is A. Only the first byte defines the netid. We can find the network address by replacing the hostid bytes ( ) with 0s. Therefore, the network address is

29 Example 6 Given the address , find the network address. Solution The class is B. The first 2 bytes defines the netid. We can find the network address by replacing the hostid bytes (17.85) with 0s. Therefore, the network address is

30 Example 7 Given the network address , find the class. Solution The class is A because the netid is only 1 byte.

31 Note: A network address is different from a netid. A network address has both netid and hostid, with 0s for the hostid.

32 Figure Sample internet

33 Note: IP addresses are designed with two levels of hierarchy.

34 Figure A network with two levels of hierarchy

35 Figure A network with three levels of hierarchy (subnetted)

36 Figure Addresses in a network with and without subnetting

37 Table 19.1 Default masks Class In Binary In Dotted- Decimal Using Slash A /8 B /16 C /24

38 Note: The network address can be found by applying the default mask to any address in the block (including itself). It retains the netid of the block and sets the hostid to 0s.

39 Example 8 A router outside the organization receives a packet with destination address Show how it finds the network address to route the packet. Solution The router follows three steps: 1. The router looks at the first byte of the address to find the class. It is class B. 2. The default mask for class B is The router ANDs this mask with the address to get The router looks in its routing table to find out how to route the packet to this destination. Later, we will see what happens if this destination does not exist.

40 Figure Subnet mask

41 Example 9 A router inside the organization receives the same packet with destination address Show how it finds the subnetwork address to route the packet. Solution The router follows three steps: 1. The router must know the mask. We assume it is /19, as shown in Figure The router applies the mask to the address, The subnet address is The router looks in its routing table to find how to route the packet to this destination. Later, we will see what happens if this destination does not exist.

42 "% $$ Binary Representation Dotted Decimal IP address Subnet mask Bitwise AND o f address and mask (resultant network/subnet number) Subnet number Host number

43 Figure NAT Network Address Translation Figure Address translation

44 Figure Translation

45 Table 19.3 Five-column translation table Private Address Private Port External Address External Port Transport Protocol TCP TCP

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51 Figure 20.9 Example of checksum calculation

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53 Figure 20.1 Protocols at network layer

54 Figure 20.2 ARP operation An ARP request is broadcast; an ARP reply is unicast.

55 Figure 20.3 ARP packet

56 Figure 20.5 Four cases using ARP

57 Example 1 A host with IP address and physical address B has a packet to send to another host with IP address and physical address A46EF45983AB. The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet frames. Solution

58 Figure 20.6 Example 1

59 20.3 ICMP Internet Control Message Protocol ICMP always reports error messages to the original source.

60 Figure Error-reporting messages

61 Figure Query messages

62 20.4 IPv6 IPv6 Addresses Categories of Addresses IPv6 Packet Format Fragmentation ICMPv6 Transition

63 Figure IPv6 address

64 Figure Format of an IPv6 datagram

65 Figure Three transition strategies

66 Figure Three transition strategies

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68 Figure Header translation