Transcript PPT
IP: Routing and Subnetting Network Protocols and Standards Autumn 2004-2005 Oct 28, 2004 CS573: Network Protocols and Standards 1 Issues in Addressing A large corporate/campus environment Large number of Local Area Networks If each physical network is assigned a network number: Oct 28, 2004 Some with fewer than 256 hosts Some with more than 256 hosts Immense administrative overhead to manage a large number of network addresses Routing tables in routers become extremely large (one entry for each physical network) Insufficient number of class B prefixes to cover medium sized networks (having more than 256 hosts) CS573: Network Protocols and Standards 2 Subnetting Solution: Provide the campus with a single class B network Oct 28, 2004 Give freedom to the campus network admin to allocate host numbers to hosts From outside, the whole campus is simply known by the class B network ID Inside, there may be a hierarchy that remains transparent to the outside world CS573: Network Protocols and Standards 3 Subnetting Consider a class B network Oct 28, 2004 How to allocate host numbers to hosts? A single LAN is out of question If host numbers are assigned randomly, i.e., without any hierarchy, the routers inside the network will have to deal with large tables – one entry per host Thus, a hierarchical structure is required CS573: Network Protocols and Standards 4 Subnetting H H H H R R R Physical Network (Subnet 2) H Physical Network (Subnet 3) R Physical Network (Subnet 1) H H R Physical Network (Subnet 4) H H Oct 28, 2004 H H H CS573: Network Protocols and Standards 5 Subnetting Network 138.10.1.0 H1 Internet R 138.10.1.1 H2 138.10.1.2 Network 138.10.2.0 R is not a Proxy ARP router! H3 138.10.2.1 Subnet 1 Subnet 2 H4 138.10.2.2 H1 wants to send an IP datagram to H3: Old addressing dictates it is a “direct delivery” With subnetting, it may become “indirect” Oct 28, 2004 CS573: Network Protocols and Standards 6 Subnetting We previously divided IP addresses in a network portion and a host portion More generally, think of a 32-bit IP address as having an Internet part and a Local part Internet part of the IP address identifies a site (possibly with many physical networks) The local portion identifies a physical network and host at that site Internet Part Internet Part Oct 28, 2004 Local Part Subnet Host CS573: Network Protocols and Standards 7 Subnetting Examples: Class B IP address Internet Part 16bits Oct 28, 2004 Subnet Host 8bits 8bits Internet Part Subnet Host 16bits 3bits 13bits CS573: Network Protocols and Standards 8 Subnet Implementation Subnet Mask: Specifies the bits of the IP address used to identify the subnet Internet Part of Address Subnet Mask (32bits) 16bits 11111111 255. 11111111 255. Internet Part of Address 11111111 255. Oct 28, 2004 16bits 11111111 255. Subnet Host 8bits 11111111 8bits 00000000 255. 0 Subnet 3bits 111 00000 224. Host 13bits 00000000 0 CS573: Network Protocols and Standards 9 Subnetting It is recommended that sites use contiguous subnet masks Avoid masks such as 11111111 11111111 11000010 11000000 When choosing a subnet mask, balance: Oct 28, 2004 Size of networks Number of networks Expected growth Ease of maintenance It is possible to use different masks in different parts of the network CS573: Network Protocols and Standards 10 Subnet Routing Conventional routing table entry (network address, next hop address) Network address format is predetermined for a given class (e.g., first 16 bits for class B addresses!) With subnetting, routing table entry becomes (subnet mask, network address, next hop address) Then compare with network address field of entries to find next hop address Oct 28, 2004 Subnet mask indicates the network address! CS573: Network Protocols and Standards 11 Subnet Routing The use of mask generalizes the subnet routing algorithm to handle all the special cases of the standard algorithm Routes to individual hosts Default route Routes to directly connected networks Routes to conventional networks (that do not use subnet addressing) Merely combine the 32-bit mask field with the 32-bit IP address Example: To install a route for: Oct 28, 2004 Individual host (Mask of all 1’s, Host IP address) Default Route (Mask of all 0’s, network address all 0’s) Class B network address (Mask of two octets of 1’s and two of 0’s) CS573: Network Protocols and Standards 12 Subnet Routing Algorithm Extract destination IP (D) from datagram Compute IP address of destination network N If N matches any directly connected network address Else Oct 28, 2004 Send datagram over that network (obviously encapsulated in a frame) For each entry in the routing table, do N* = bitwise-AND of D and subnet mask If N* equals the network address field of the entry, then route the datagram to the specified next hop CS573: Network Protocols and Standards 13 Supernet Addressing Use of many IP network addresses for a single organization Example: To conserve class B addresses, issue multiple class C address to the same organization Issue: increase in the number of entries in the routing table Solutions: Oct 28, 2004 Collapse a block of contiguous class C address into the pair: (network address, count) where network address is the smallest number in the block CS573: Network Protocols and Standards 14 Supernet Addressing It requires each block to be a power of 2 and uses bit mask to identify the size of the block Example Dotted decimal 32-bit binary equivalent Lowest: 234.170.168.0 11101010 10101010 10101000 00000000 Highest: 234.170.175.255 11101010 10101010 10101111 11111111 A block of 2048 addresses 32-bit mask is 11111111 11111111 11111000 00000000 Do we really need address classes when we have masks? Oct 28, 2004 Answer: NO CIDR (Classless Inter Domain Routing) CS573: Network Protocols and Standards 15 Supernet Addressing In the router, the entry consists of: Oct 28, 2004 The lowest address and the 32-bit mask A block of addresses can be subdivided, and separate route can be entered for each subdivision When looking up a route, the routing software uses a longest-match paradigm to select a route CS573: Network Protocols and Standards 16 IPv6 Motivation Limited address space Support for new applications Oct 28, 2004 Multimedia streams, for example Security Extensibility CS573: Network Protocols and Standards 17 Features of IPv6 Larger addresses Flexible header format Set of optional headers Support for flow identification 128 bit addresses Needed in resource allocation for multimedia streams Provision for protocol extension Oct 28, 2004 CS573: Network Protocols and Standards 18