IP: Routing and Subnetting

Transcription

1 IP: outing and Network Protocols and Standards Autumn Oct 28, 2004 CS573: Network Protocols and Standards 1 Issues in Addressing A large corporate/campus environment Large number of Local Area Networks Some with fewer than 256 hosts Some with more than 256 hosts If each physical network is assigned a network number: Immense administrative overhead to manage a large number of network addresses outing 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) Oct 28, 2004 CS573: Network Protocols and Standards 2 CS573: Network Protocols and Standards 1

2 Solution: Provide the campus with a single class B network 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 Oct 28, 2004 CS573: Network Protocols and Standards 3 Consider a class B network ow 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 Oct 28, 2004 CS573: Network Protocols and Standards 4 CS573: Network Protocols and Standards 2

3 Physical Network (Subnet 1) Physical Network (Subnet 3) Physical Network (Subnet 2) Physical Network (Subnet 4) Oct 28, 2004 CS573: Network Protocols and Standards 5 Network Subnet 1 Internet Network Subnet 2 is not a Proxy AP router! wants to send an IP datagram to 3: Old addressing dictates it is a direct delivery With subnetting, it may become indirect Oct 28, 2004 CS573: Network Protocols and Standards 6 CS573: Network Protocols and Standards 3

4 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 Local Part Internet Part Subnet ost Oct 28, 2004 CS573: Network Protocols and Standards 7 Examples: Class B IP address Internet Part Subnet ost 16bits 8bits 8bits Internet Part Subnet ost 16bits 3bits 13bits Oct 28, 2004 CS573: Network Protocols and Standards 8 CS573: Network Protocols and Standards 4

5 Subnet Implementation Subnet Mask: Specifies the bits of the IP address used to identify the subnet Internet Part of Address Subnet ost Subnet Mask (32bits) 16bits 8bits 8bits Internet Part of Address Subnet ost 16bits 3bits 13bits Oct 28, 2004 CS573: Network Protocols and Standards 9 It is recommended that sites use contiguous subnet masks Avoid masks such as When choosing a subnet mask, balance: Size of networks Number of networks Expected growth Ease of maintenance It is possible to use different masks in different parts of the network Oct 28, 2004 CS573: Network Protocols and Standards 10 CS573: Network Protocols and Standards 5

6 Subnet outing 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 f i fi d h dd Oct 28, 2004 CS573: Network Protocols and Standards 11 Subnet outing The use of mask generalizes the subnet routing algorithm to handle all the special cases of the standard algorithm outes to individual hosts Default route outes to directly connected networks outes 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: Individual host (Mask of all 1 s, ost IP address) Default oute (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) Oct 28, 2004 CS573: Network Protocols and Standards 12 CS573: Network Protocols and Standards 6

7 Subnet outing Algorithm Extract destination IP (D) from datagram Compute IP address of destination network N If N matches any directly connected network address Send datagram over that network (obviously encapsulated in a frame) Else 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 Oct 28, 2004 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: Collapse a block of contiguous class C address into the pair: (network address, count) where network address is the smallest number in the block Oct 28, 2004 CS573: Network Protocols and Standards 14 CS573: Network Protocols and Standards 7

8 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: ighest: A block of 2048 addresses 32-bit mask is Do we really need address classes when we have masks? Answer: NO CID (Classless Inter Domain outing) Oct 28, 2004 CS573: Network Protocols and Standards 15 Supernet Addressing In the router, the entry consists of: 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 Oct 28, 2004 CS573: Network Protocols and Standards 16 CS573: Network Protocols and Standards 8

9 IPv6 Motivation Limited address space Support for new applications Multimedia streams, for example Security Extensibility Oct 28, 2004 CS573: Network Protocols and Standards 17 Features of IPv6 Larger addresses 128 bit addresses Flexible header format Set of optional headers Support for flow identification Needed in resource allocation for multimedia streams Provision for protocol extension Oct 28, 2004 CS573: Network Protocols and Standards 18 CS573: Network Protocols and Standards 9