Oct 26, 2004CS573: Network Protocols and Standards1 IP: Routing and Subnetting Network Protocols and Standards Autumn

Oct 26, 2004CS573: Network Protocols and Standards2 Routing IP Datagram Direct Delivery (i.e., not involving routers): Transmission of an IP datagram between two machines on a single physical network does not involve routers The sender encapsulates the datagram in a physical frame, binds the destination IP address to a physical hardware address (using ARP), and sends the resulting frame directly to the destination The two machines are known to be on the same network because they have the same network identifier Example: A sends IP Datagram to B Router ABC

Oct 26, 2004CS573: Network Protocols and Standards3 Routing IP Datagram Indirect delivery (i.e. through intermediate routers) Host performs routing decisions based on routing table indicating “next hop” “Next hop” refers to next router IP address on this network, via which the destination is reached Routing decisions are made based on network prefixes (not full IP address) The sender encapsulates the datagram in a frame with the router’s physical destination address (which is found by means of ARP).

Oct 26, 2004CS573: Network Protocols and Standards4 Direct and Indirect Routing Host A Host B Router Internet Host C Direct Routing: Packets sent directly using MAC address of A Indirect Routing: Packets sent to the MAC address of the router. At the IP level, B is the source and C is the destination B wants to send packets to A and C!

Oct 26, 2004CS573: Network Protocols and Standards5 IP Routing Decisions Network Network Network Network R1 R2 R Routing Table of R2 To Reach Hosts on Network Next Hop Address Direct Delivery Direct Delivery

Oct 26, 2004CS573: Network Protocols and Standards6 IP Routing Algorithm Router receives an IP datagram with network portion N and destination D If N is directly connected Transmit on that network Else If host specific entry for D exists Use next hop in that entry Else If route entry for N exists Use next hop in that entry Else If default route for next hop exists Use default route for next hop Else Declare error

Oct 26, 2004CS573: Network Protocols and Standards7 Routing Within Same Network Consider a small company with a single LAN to which a class C network address has been assigned The company is interested in adding another small physical network (connected to old network through a router) with a few hosts Question: Could this company assign these hosts IP addresses from the same C class network? i.e., could the two LANs share the same class C network address?

Oct 26, 2004CS573: Network Protocols and Standards8 Proxy ARP Used to allow two physical networks to share the same IP network prefix Router R’s table is configured manually to route between these two networks Router R answers ARP requests on each network for hosts on the other network, giving its own hardware address as the target address Main Router ABC Main Network To Internet Hidden Network ED Router R

Oct 26, 2004CS573: Network Protocols and Standards9 Proxy ARP Advantage of Proxy ARP Router Can be added without disturbing the routing table in other hosts or routers on that network Disadvantages: Does not generalize to complex network topologies (does not scale) Does not support a reasonable form of routing. (relies on network managers to maintain tables of machines and addresses manually) Issues: Several IP addresses map to the same physical address. How to distinguish between a legitimate Proxy ARP router and spoofing?

Oct 26, 2004CS573: Network Protocols and Standards10 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 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)

Oct 26, 2004CS573: Network Protocols and Standards11 Subnetting 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 26, 2004CS573: Network Protocols and Standards12 Subnetting Consider a class B network 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

Oct 26, 2004CS573: Network Protocols and Standards13 Subnetting Physical Network (Subnet 1) Physical Network (Subnet 2) Physical Network (Subnet 3) Physical Network (Subnet 4) R R R R R H H H H H HH H H H H H

Oct 26, 2004CS573: Network Protocols and Standards14 Subnetting R Internet H1H2 H4H3 Network Network H1 wants to send an IP datagram to H3: Old addressing dictates it is a “direct delivery” With subnetting, it may become “indirect” R is not a Proxy ARP router! Subnet 1 Subnet 2

Oct 26, 2004CS573: Network Protocols and Standards15 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 PartLocal Part Internet PartSubnetHost

Oct 26, 2004CS573: Network Protocols and Standards16 Subnetting Examples: Class B IP address Internet PartSubnetHost 16bits 8bits 8bits Internet PartSubnetHost 16bits3bits 13bits

Oct 26, 2004CS573: Network Protocols and Standards17 Subnet Implementation Subnet Mask: Specifies the bits of the IP address used to identify the subnet Internet Part of AddressSubnetHost 16bits 8bits 8bits Internet Part of AddressSubnetHost Subnet Mask (32bits) 16bits3bits 13bits

Oct 26, 2004CS573: Network Protocols and Standards18 Subnetting 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 26, 2004CS573: Network Protocols and Standards19 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 Subnet mask indicates the network address!

Oct 26, 2004CS573: Network Protocols and Standards20 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: 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)

Oct 26, 2004CS573: Network Protocols and Standards21 Subnet Routing 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 26, 2004CS573: Network Protocols and Standards22 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 26, 2004CS573: Network Protocols and Standards23 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 decimal32-bit binary equivalent Lowest: Highest: A block of 2048 addresses 32-bit mask is Do we really need address classes when we have masks? Answer: NO  CIDR (Classless Inter Domain Routing)

Oct 26, 2004CS573: Network Protocols and Standards24 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 26, 2004CS573: Network Protocols and Standards25 IPv6 Motivation Limited address space Support for new applications Multimedia streams, for example Security Extensibility

Oct 26, 2004CS573: Network Protocols and Standards26 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