IP Addressing and Routing

Transcription

1 IP Addressing and Routing 1 Basic IP Addressing Each host connected to the Internet is identified by a unique IP address. An IP address is a 32-bit quantity. Expressed as a dotted-decimal notation W.X.Y.Z. Consists of two logical parts: A network number A host number This partition defines the IP address classes. 2 1

2 IP Address Classes There are five defined IP address classes. Class A Class B Class C Class D Class E UNICAST UNICAST UNICAST MULTICAST RESERVED There are some special-purpose IP addresses also. 3 Class Address Range Highorder bits Network bits Host bits A B C D E

3 Special-Purpose IP Addresses Address Range Purpose Unknown network, commonly represents default Reserved for private use Reserved for loopback / local address Reserved for private use Reserved for private use Limited broadcast 5 The class-based addressing is also known as the classful model. Different network classes lend themselves to different network configurations. Different network-to-hosts ratio. 6 3

4 Some Conventions Within a particular network (Class A, B or C), the first and last addresses serve special functions. The first address represents the network number (for example, ). The last address represents the directed broadcast address of the network (for example, ). 7 IP Subnetting 8 4

5 IP Subnet Basic concept: A subset of a class A, B or C network. IP addresses that do not use subnets consists of a network portion, and a host portion. Represents a static two-level hierarchical addressing model. 9 IP Subnet (contd.) IP subnets introduces a third level of hierarchy. a network portion a subnet portion a host portion Allow more efficient (and structured) utilization of the addresses. Uses network masks. 10 5

6 Natural Masks Network mask is applied to a class A network In binary, the mask is a series of contiguous 1 s followed by a series of contiguous 0 s Network portion Host portion 11 Natural Masks (contd.) Provide a mechanism to split the IP address into a network portion of 10, and a host portion of 20. Decimal Binary IP address: Mask: Network Host 12 6

7 Natural Masks (contd.) Class A, B and C addresses Have fixed division of network and host portions. Can be expressed as masks. Called natural masks. Natural Masks Class A :: Class B :: Class C :: Creating Subnets using Masks Masks are very flexible. Using masks, networks can be divided into smaller subnets. How? By extending the network portion of the address into the host portion. Advantage gained: We can create a large number of subnets from one network. Can have less number of hosts per network. 14 7

8 Example: Subnets Network mask is applied to a class A network This divides the IP address into a network portion of 10, a subnet portion of 5, and a host portion of 20. The mask borrows a portion of the host space, and applies it to network space. 15 Subnets (contd.) What happens? Initially it was a single large Class A network ( hosts). We have now split the network into 256 subnets. From to The hosts pet subnet decreases to 65,

9 Subnets (contd.) Decimal Binary IP address: Mask: Network Subnet Host 17 Default Mask and Subnet mask IP Address Default Mask AND Network Address IP Address AND Subnet Mask Network Address : :

10 Subnets vrs Multiple Address Classes Subnets Management of subnets is done by local network administrator. Single entry in external router tables. Multiple Address Classes Multiple entries in external router tables. Additional overhead on the backbone (external) routers. 19 Comparison R R SUBNETS R R R MULTIPLE ADDRESS CLASSES R 20 10

11 Variable Length Subnet Mask (VLSM) 21 Variable Length Subnet Masks (VLSM) Basic concept The same network can be configured with different masks. Can have subnets of different sizes. Allows better utilization of available addresses

12 Example: VLSM Suppose we are assigned a Class C network To be divided into three subnets. Corresponding to three departments. With 110, 45 and 50 hosts respectively. D1 (110) D2 (45) D3 (50) 23 The Example (contd.) Available subnet options The network mask will be the Class C natural mask Subnet masks of the form X Can be used to divide the network into more subnets

13 The Subnet Options X X (in binary) No. of Subnets No. of Hosts Cannot satisfy the requirements. 25 The VLSM Option Basic concept: Use the mask to divide the network address into two subnets with 128 hosts each (.0 to.127) (.128 to.255) 26 13

14 The VLSM Option (contd.) Next subnet the second.128 subnet using a mask of Creates two subnets, 64 hosts each (.128 to.191) (.192 to.255) 27 The VLSM Option (contd.) Mask: (.0 to.127) (.128 to.255) (.128 to.191) Mask: (.192 to.255) 28 14

15 Interface 1 :: 128 hosts Network number: Network mask: Address: Interface 2 :: 64 hosts Network number: Network mask: Address: Interface 3 :: 64 hosts Network number: Network mask: Address: Hosts E2 64 Hosts E3 ROUTER E4 64 Hosts Interface E3 :: 64 hosts Network number: Network mask: Address range: Interface E2 :: 128 hosts Network number: Network mask: Address range: Interface E4 :: 64 hosts Network number: Network mask: Address range:

16 VLSM :: Current Status All routing protocols do not support VLSM. Routing Information Protocol version 1 (RIP-1) do not carry network masks in routing updates. RIP-1 cannot implement VLSM. The following protocols support VLSM: Open Shortest Path First (OSPF) RIP-2 Enhanced IGRP (EIGRP) 31 Classless Internet Domain Routing (Supernetting) 32 16

17 Classless Inter-Domain Routing (CIDR) The size of the global routing tables have grown very fast in recent years. Caused routers to become saturated. Limits to processing power and available memory. Size of the tables have doubled every 10 months or so, between 1991 and Without any remedial measure, the routing tables would have grown to about 80,000 routes in But early 2000 data shows that the size was around 76,000. Why this reduction? Planned IP address allocation. CIDR

18 Growth of Internet Routing Tables '88 '94 '96 '98 '00 Routing Table Size Year 35 CIDR: Introduction CIDR is a new concept to manage IP networks. Classless Inter Domain Routing. No concept of class A, B, C networks. Reduces sizes of routing tables

19 CIDR: Basic Idea An IP address is represented by a prefix, which is the IP address of the network. It is followed by a slash, followed by a number M. M: number of leftmost contiguous bits to be used for the network mask. Example: / CIDR: An Important Rule The number of addresses in each block must be a power of 2. The beginning address in each block must be divisible by the number of addresses in the block. A block that contains 16 addresses cannot have beginning address as But the address is possible

20 Example: CIDR An organization is allotted a block with beginning address: / 29 What is the range of the block? Start addr: End addr: There are 8 addresses in the block. 39 Example Suppose Company A needs IP addresses for 1000 machines Assign 4 contiguous Class C address blocks (last 8 bits 0) 40 20

21 Supernet: Address : Netmask: (last 10 bits 0) Also written as: /22 22 denotes size of network portion. Also called prefix. Routing done by prefix 41 Advantages Routing table at higher levels will have only one entry for the 4 networks. In classful addressing (that did not recognize masks), would have required 4 entries for the 4 networks. Possible only due to contiguous allocation. Higher level routers can just send it to lower level routers (in this case company A s router) using one entry only. Lower level router will distinguish

22 Routing table at all higher level routers: /22 - send to host X (next hop on way to Company A s router RA) Routing table at RA: /24 send to router of first net /24 send to router of second net /24 send to router of third net /24 send to router of fourth net RA 43 Routers always do longest prefix match. If two entries match, longest match is taken. Example: two entries in table: one for /16 and one for /24. If address is , second entry will be used even though it matches both

23 Recent Trend Move on to CIDR addressing. Existing classful networks can also be represented using this notation. Class A: W.X.Y.Z / 8 Class B: W.X.Y.Z / 16 Class C: W.X.Y.Z / 24 Recent routers support CIDR