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IP: Routing and Subnetting

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IP: Routing and Subnetting

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  1. IP: Routing and Subnetting Network Protocols and Standards Autumn 2004-2005 CS573: Network Protocols and Standards

  2. 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 A B C CS573: Network Protocols and Standards

  3. 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). CS573: Network Protocols and Standards

  4. Direct and Indirect Routing B wants to send packets to A and C! Host A 204.240.18.10 Internet 204.240.18.1 Router 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 Host B 204.240.18.20 Host C 36.14.0.200 CS573: Network Protocols and Standards

  5. Network 10.0.0.0 Network 40.0.0.0 Network 20.0.0.0 Network 30.0.0.0 IP Routing Decisions 10.0.0.5 40.0.0.7 20.0.0.6 30.0.0.6 20.0.0.5 R3 R1 R2 30.0.0.7 Routing Table of R2 CS573: Network Protocols and Standards

  6. 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 CS573: Network Protocols and Standards

  7. 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? CS573: Network Protocols and Standards

  8. 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 To Internet Main Network A B C Router R D E Hidden Network CS573: Network Protocols and Standards

  9. 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? CS573: Network Protocols and Standards

  10. 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) CS573: Network Protocols and Standards

  11. 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 CS573: Network Protocols and Standards

  12. 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 CS573: Network Protocols and Standards

  13. Subnetting H H H H Physical Network (Subnet 3) Physical Network (Subnet 1) R H H R R H R Physical Network (Subnet 4) Physical Network (Subnet 2) R H H H H H CS573: Network Protocols and Standards

  14. Subnetting Network 138.10.1.0 Subnet 1 H1 H2 Internet R 138.10.1.1 138.10.1.2 Network 138.10.2.0 Subnet 2 H3 H4 R is not a Proxy ARP router! 138.10.2.1 138.10.2.2 H1 wants to send an IP datagram to H3: Old addressing dictates it is a “direct delivery” With subnetting, it may become “indirect” CS573: Network Protocols and Standards

  15. 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 (note: physical network == extended LAN) Internet Part Local Part Internet Part Subnet Host CS573: Network Protocols and Standards

  16. Subnetting Examples: Class B IP address Internet Part Subnet Host 16bits 8bits 8bits Internet Part Subnet Host 16bits 3bits 13bits CS573: Network Protocols and Standards

  17. Subnet Implementation Subnet Mask: Specifies the bits of the IP address used to identify the subnet Internet Part of Address Subnet Host 16bits 8bits 8bits 11111111 11111111 11111111 00000000 Subnet Mask (32bits) 255. 255. 255. 0 Internet Part of Address Subnet Host 16bits 3bits 13bits 11111111 11111111 111 00000 00000000 255. 255. 224. 0 CS573: Network Protocols and Standards

  18. Subnetting • It is recommended that sites use contiguous subnet masks • Avoid masks such as 11111111 11111111 11000010 11000000 • 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 CS573: Network Protocols and Standards

  19. 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! CS573: Network Protocols and Standards

  20. 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) CS573: Network Protocols and Standards

  21. 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 CS573: Network Protocols and Standards

  22. 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 CS573: Network Protocols and Standards

  23. 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: 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? • Answer: NO  CIDR (Classless Inter Domain Routing) CS573: Network Protocols and Standards

  24. 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 CS573: Network Protocols and Standards

  25. IPv6 • Motivation • Limited address space • Support for new applications • Multimedia streams, for example • Security • Extensibility CS573: Network Protocols and Standards

  26. 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 CS573: Network Protocols and Standards