# COMP9332 Network Routing & Switching

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1 COMP9332 Network Routing & Switching IPv4 Addressing 1

3 IP addressing basics The Internet is used to move data from host to host All devices connected to the Internet must have a globally unique IP address No two devices can have the same public IP address This address can be permanent or temporary IPv4 addresses are 32 bits (= 4 octets) long This gives 2^32 ~ 4.29 billion addresses 3

4 Notation IPv4 addresses can be written using the following notation Binary Dotted Decimal Hexadecimal 4

5

6 Figure 4-1 Dotted-decimal notation 6

7 D EA 0x75951DEA 7

8 Exercises Exercise 1: Change the following IP address from binary notation to dotted-decimal notation Exercise 2: Change the following IP address from dotteddecimal notation to binary notation

9 Solutions Exercise 1: Exercise 2:

10 Classful addressing IP addresses were divided into 5 classes: A,B,C,D and E This is the original scheme known as classful addressing From mid-90 s, classless addressing is introduced However, classful addressing is still used 10

11 Finding the class in binary notation Example: Any address whose first bit is 0 belongs to class A. Any address whose first 2 bits are 10 belongs to class B etc. 11

12 Exercises 1. Find the class of these IP addresses: a) b) How many class B addresses are there altogether? 3. What is the range of class B addresses? Answer this by giving the first and last class B addresses in dotted decimal notation. 12

13 Solution 1. a) First 3 bits are 110 -> Class C. b) First 2 bits are 10 -> Class B. 2. Class B addresses: the first two bits are 10 then followed by 30 bits of 1/0 -> 2^30 addresses 13

14 3. The first and last class B addresses in binary are: In dotted decimal notation, they are: and

15 Finding the class in dotted decimal notation Given an IP address in dotted decimal notation, we can identify its class by looking at the first byte. Example: The first byte of a class C address is of the form 110x xxxx where x is either 0 or 1, which means that it ranges from (192) to (223) 15

16 Finding the class in decimal notation 16

17 IP addresses are hierarchical Net1.Host1 Net1.Host2 Net2.Host1 Net2.Host2 Interface1 Routing table: Net1: Net2: Interface2 Interface1 Interface2 Routing is based on network address. 17

18 Netid and hostid IP addresses in classes A,B and C are divided into netid and hostid Netid: Identifying the network Hostid: Identifying a host within the network Hosts within a network Have the same netid But different hostid 18

19 Netid and hostid 19

20 Classes and blocks All addresses within a block have the same netid. For the class A block with netid = 0, the addresses in the block are: netid hostid

21 Classes and blocks Class A is divided into 128 blocks Each block has a different netid 1st block: to (netid = 0) 2nd block: to (netid = 1) Last block: to (netid = 127) Network address: the first address of the block 21

22 Blocks in class A 22

23 Blocks in class B 23

24 Blocks in class C 24

25 Use of addresses Classes A, B and C addresses can be assigned to hosts, router ports etc They are also known as unicast addresses Class D addresses are for multicast Multicast: One sender, multiple recipients Class E addresses are reserved for special purposes 25

26 Network addresses The network address is the first address in the block The network address defines the network to the rest of the Internet Routers route packets based on network address Given the network address, we can find the class of the address and the range of the address in the block 26

27 Exercise Given the network address , find the class, the block, and the range of the addresses. 27

28 Solution Solution: The class is B because the first byte is between 128 and 191. The block has a netid of The addresses range from to

29 Finding netid from IP address Given an IP address, we can identify the network address by Finding which class it belongs to and then obtain the netid part E.g. Given » This is class B. The netid is the first 2 bytes. The network address is An alternative is to use network mask 29

30 Network mask Let & denote the bitwise AND operation Example: 1100 & 1010 = 1000 A network mask is 32 bit binary number Often written in dotted decimal notation Is chosen such that the following relation holds Network address = IP address & Network mask 30

31 Network mask (cont d) The network mask is different for each class Class A» or Class B» or Class C» or Essentially: 1 indicates that the bit is a netid bit 0 indicates that the bit is a hostid bit 31

32 Example A host has IP address and network mask , find the network to which this host belongs. Method 1: & = =

33 Example (cont d) Let X denote a byte, then X & 0 = 0 and X & 255 = X Method 2: & =

34 Why network mask? A host is specified by two attributes An IP address A network mask We can find the netid from these two attributes without finding which class the address belongs to This makes it easier to program You ll see the importance of network mask later when we study subnetting and classless addressing 34

35 Multihomed devices Strictly speaking, IP addresses are assigned to network interfaces. 35

36 Special IP addresses Some IP addresses have been assigned special meaning They are not meant to be assigned to individual hosts 36

39 Assignable IP addresses These IP addresses cannot be assigned to individual hosts Netid + All-zero-hostid Netid + All-one-hostid Example, although each Class C network, has 256 different hostid s, the number of assignable addresses is only

41 Example of this host on this address To find out more: BOOTP and DHCP protocols (Chap 17) 41

42 Example of specific host on this network 42

43 Example of loopback address 43

44 Private addresses Some IP addresses are reserved to be used as private addresses A host can use private address if it is not directly connected to the Internet To connect to the Internet, Network Address Translation is required The following netids are designated as private 10, ,

45 Sample internet 45

46 Sample Internet - commentary The Internet is organised into networks Routers interconnect these networks into the Internet All hosts within the same network have the same netid 46

47 Motivation for subnetting Classful addressing means network must be of size: (2^16) for Class B and 256 (2^8) for Class C If an organisation wants to have 4 networks each with 300 hosts, then it needs 4 Class B networks More than 200,000 addresses are wasted Subnetting was introduced in 1980s to solve this problem In subnetting, a network is divided into subnets (short for sub-networks) 47

48 A network which is not subnetted 48

49 Example of a subnetted network External Router 49

50 Transparency of subnetting In the example on the previous page, only R1, R2 and R3 need to know about the subnets Each subnet requires an entry in their routing table Routers external to the network do not need to know about subnetting The external router requires only an entry for the network (i.e ) in its routing table 50

51 Subnetid is an address in the subnet (subnet number) Before subnetting netid After subnetting netid subnetid hostid 51

52 Subnet mask When we configure a host, we need to specify The IP address of the host, and The subnet mask The subnet mask has the same role as network mask 1 indicates that bit is a netid or subnetid bit 0 indicates that bit is a hostid bit 52

53 Subnet mask example Given the following netid, subnetid and hostid (Note: 1st 2 bytes are in dotted decimal, last 2 bytes in binary) netid subnetid hostid The corresponding subnet mask is In dotted decimal notation, it is

55 55

56 Short-Cut Method ** If the byte in the mask is 255, copy the byte in the address. ** If the byte in the mask is 0, replace the byte in the address with 0. ** If the byte in the mask is neither 255 nor 0, we write the mask and the address in binary and apply the AND operation. 56

57 Exercise What is the subnet address if the IP address is and the mask is ? 57

58 Example 2 58

59 Comparison of a default mask and a subnet mask Since subnetid has 3 bits, the number of subnets allowed is 2^3. Since the hostid has 13 bit, the number of addresses per subnet is 2^13. 59

60 Address range in a subnet A subnet has an subnet address of , find the address range in the subnet if its subnet mask is

61 Solution If the network mask is (netid - 16bits, subnetid - 3bits) The address range is: netid subnetid hostid to If the network mask is (netid - 16bits, subnetid - 4bits) The address range is: netid subnetid hostid to 61

62 Notes on subnetting To define a subnet, you must define both Subnet address and Subnet mask It s not enough to give only the subnet address 62

63 A company is granted the site address (class C). The company needs six subnets. Design the subnets. The number of 1s in the default mask is 24 (class C).

64 The company needs six subnets. This number 6 is not a power of 2. The next number that is a power of 2 is 8 (2 3 ). We need 3 more 1s in the subnet mask. The total number of 1s in the subnet mask is 27 (24 + 3). The total number of 0s is 5 (32-27). The mask is

65 or The number of subnets is 8. The number of addresses in each subnet is 2 5 (5 is the number of 0s) or 32.

66 The address range for the subnets are: The subnet number is the first address in the subnet. 66

67 Variable-length subnetting 67

68 Supernetting Background Class B addresses were running out There were still plenty of class C addresses but each class C network has only 256 addresses If an organisation wants 1000 addresses, supernetting allows 4 class C networks to be merged to form a supernet with 1024 addresses 68

69 A supernetwork

70 Address assignment If a subnet consists of 100 Class C networks and if these addresses are randomly chose The routers external to the supernet requires 100 entries (one for each Class C network) for the supernet It would be desirable if only 1 entry is required This can be achieved by carefully assigning addresses 70

71 Address assignment rules The number of blocks must be a power of 2 (1, 2, 4, 8, 16,...). The blocks must be contiguous in the address space (no gaps between the blocks). The third byte of the first address in the superblock must be evenly divisible by the number of blocks. In other words, if the number of blocks is N, the third byte must be divisible by N. 71

72 Exercise A company needs 600 addresses. Which of the following set of class C blocks can be used to form a supernet for this company? a) b) c) d)

73 Solution a) Not acceptable. #blocks not a power of 2. b) Not acceptable. Not contagious. c) Not acceptable. 3rd byte of 1st address not divisible by 4. d) Acceptable. 73

74 Specifying a supernet Analogous to a subnet, a supernet is specified by A supernet address A supernet mask 74

75 Comparison of subnet, default, and supernet masks 75

76 We need to make a supernetwork out of 16 class C blocks. What is the supernet mask? We need 16 blocks. For 16 blocks we need to change four 1s to 0s in the default mask. So the mask is or

77 A supernet has a first address of and a supernet mask of How many blocks are in this supernet and what is the range of addresses? The supernet mask has 21 1s. The default mask has 24 1 s. Since the difference is 3, there are 2 3 or 8 blocks in this supernet. The blocks are , ,, The first address is The last address is

78 Classless addressing Classful addressing: The number of addresses in a network can only be 2^8, 2^16 or 2^24 A supernet consisting of multiple Class C networks can have 256 * 2^n (n=2, 7) addresses The number of addresses in a classless network can be any number as long as it is a power of 2 Classless network is part of the Classless Interdomain Routing Protocol (CIDR) 78

79 Beginning address The addresses in a classless network must be chosen carefully so that only one entry in the routing table is required The beginning address of a classless network must be divisible by the number of addresses in the network 79

80 Beginning address (cont d) The IP address in dotted decimal a.b.c.d is actually the decimal number: a b c d E.g. to check whether an IP address is divisible by 16, we only need to check whether the last byte is divisible by 16 80

81 Which of the following can be the beginning address of a block that contains 16 addresses? The address is eligible because 32 is divisible by 16. The address is eligible because 80 is divisible by 16.

82 Which of the following can be the beginning address of a block that contains 1024 addresses? To be divisible by 1024, the rightmost byte of an address should be 0 and the second rightmost byte must be divisible by 4. Only the address meets this condition.

83 Classless networks A classless network is specified by A network address A mask Since a mask consists of a number of 1 s at the left followed by a number of 0 s, instead of specifying the mask in dotted decimal, we can specify the number of 1 s 83

84 CIDR notation A classless network is usually written as A.B.C.D/n n specifies the number of 1 s in the mask This is known as slash notation or CIDR notation The first n bits of A.B.C.D is known as the prefix and n is known as the prefix length The last (32-n) bits are known as the suffix 84

85 A small organization has the network /29. What is the range of the block? The beginning address is To find the last address we keep the first 29 bits and change the last 3 bits to 1s. Beginning: Ending : There are only 8 addresses in this block.

86 What is the network address if one of the addresses is /27? The prefix length is 27, which means that we must keep the first 27 bits as is and change the remaining bits (5) to 0s. The 5 bits affect only the last byte. The last byte is Changing the last 5 bits to 0s, we get or 64. The network address is /27.

87 Subnetting classless network A classless network also be subnetted Example: An organization is granted the block /26. The organization needs to have four subnets. What are the subnet addresses and the range of addresses for each subnet? 87

88 Since the prefix length is 26. This means the last 6 bits are available as hostid, the total number of addresses in the block is 64 (2 6 ). If we create four subnets, each subnet will have 16 addresses. 88

89 Let us first find the subnet prefix (subnet mask). We need four subnets, which means we need to add two more 1s to the site prefix. The subnet prefix is then /28. Subnet 1: /28 to /28. Subnet 2 : /28 to /28. Subnet 3: /28 to /28. Subnet 4: /28 to /28.

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