TCOM 509 – Internet Protocols (TCP/IP) Lecture 06_b Subnetting,Supernetting, CIDR IPv6 Instructor: Dr. Li-Chuan Chen Date: 10/06/2003 Based in part upon slides of Prof. J. Kurose (U Mass)

IP Addresses Review IP address is 32-bit in IPv4. IP address are designed with two levels of hierarchy, network portion and host portion. Classful addressing inefficient: Class A and Class B waste many address spaces for each network. Solution: use another level of hierarchy, subnetting. Further divide a network into smaller networks called subnets. netidhostid netidhostid2subnetid Number of Subnets = 2 subnetid Number of Hosts per subnet = 2 hostid2

Subnet Mask Class A: Class B: Class C: ’s 16 1’s 24 1’s Class A subnet mask: Class B subnet mask: Class C subnet mask:

Subnet Example What is the subnetwork address if the destination address is and the subnet mask is ? –Apply the AND operation  subnetwork address

Subnet Example A company has the Class B address The company needs 1000 subnets. Design the subnets. (Explain in class.)

Variable Length Subnet Mask Suppose a site with class C address and needs to have 5 subnets with hosts as follows: 40, 40, 30, 30. The site cannot use subnet mask of 26 bits. Why? Solution: use variable length subnet mask (vlsm). –First uses the 26-bit subnet mask ( ) to divide the network into 4 subnets. –Then it applies with 27-bit subnet mask ( ) to one of the subnets to divide it further into two smaller subnets.

Supernetting Class A and B addresses are almost depleted. Class C is available, but most organization needs more than 256 hosts in the network. Solution: use supernetting. –Combine several class C networks to create a supernetwork (less number of 1’s than default mask) A supernet mask is reverse of a subnet mask Default Mask Subnet Mask Supernet Mask

Supernet Example A company needs to make a supernet out of its 8 Class C address. What is the supernet mask? A supernet has a first address of and a supernet mask of How many blocks are in this supernet? What is the range of addresses? What is the total number of addresses?

Classless Addressing What if a small home business only wants 8 addresses? Solution: –Use classless addressing: variable-length blocks that belongs to no class. –The whole address space, 232, is divided into blocks of different sizes. Rules: –Number of blocks must be power of 2. –The beginning address must be divisible by the number of addresses. prefixsuffix Boundary is flexible

Classless Example Which of the following can be the beginning address of a block that contains 16 addresses? – – – – Rememer: The beginning address must be divisible by the number of addresses.

Classless InterDomain Routing (CIDR) CIDR notation w.x.y.z/n Where n denotes the number of bits that are the same in every address in the block. Examples: –A site is given a block with the beginning address and the prefix length /30. What is the range of the block? Beginning address: Ending address:  Only 4 address in this block –What is the network address of /27? The prefix length is 27 (must keep the first 27 bits the same) and change the remaining bits to 0’s.  The network address is /27

Routing in the Internet Autonomous Systems (AS): A collection of hosts and routers that are administered by a single authority. The Global Internet consists of AS interconnected with each other: –Stub AS: small corporation: one connection to other AS’s –Multihomed AS: large corporation (no transit): multiple connections to other AS’s –Transit AS: provider, hooking many AS’s together Two-level routing: –Intra-AS: administrator responsible for choice of routing algorithm within network –Inter-AS: unique standard for inter-AS routing: BGP

Subnetting Autonomous Systems (AS): A collection of hosts and routers that are administered by a single authority. The Global Internet consists of AS interconnected with each other: –Stub AS: small corporation: one connection to other AS’s –Multihomed AS: large corporation (no transit): multiple connections to other AS’s –Transit AS: provider, hooking many AS’s together Two-level routing: –Intra-AS: administrator responsible for choice of routing algorithm within network –Inter-AS: unique standard for inter-AS routing: BGP

Why different Intra- and Inter-AS routing ? Policy: Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no policy decisions needed Scale: hierarchical routing saves table size, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance

IPv6 Initial motivation: 32-bit address space completely allocated by Additional motivation: –Efficiency - header format helps speed processing/forwarding –QoS - header changes –new “anycast” address: route to “best” of several replicated servers IPv6 datagram format: –fixed-length 40 byte header –no fragmentation allowed

IPv6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of “flow” not well defined). Next header: identify upper layer protocol for data

Other Changes from IPv4 Checksum: removed entirely to reduce processing time at each hop Options: allowed, but outside of header, indicated by “Next Header” field ICMPv6: new version of ICMP –additional message types, e.g. “Packet Too Big” –multicast group management functions

Transition From IPv4 To IPv6 Not all routers can be upgraded simultaneous –How will the network operate with mixed IPv4 and IPv6 routers? Two proposed approaches: –Dual Stack: some routers with dual stack (v6, v4) can “translate” between formats –Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

Dual Stack Approach A B E F IPv6 C D IPv4 Flow: X Src: A Dest: F data Flow: ?? Src: A Dest: F data Src:A Dest: F data A-to-B: IPv6 Src:A Dest: F data B-to-C: IPv4 B-to-C: IPv4 B-to-C: IPv6

Tunneling A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 C D IPv4 Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Src:B Dest: E Flow: X Src: A Dest: F data Src:B Dest: E A-to-B: IPv6 E-to-F: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4