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IP: Routing and Subnetting Network Protocols and Standards Autumn 2004-2005 Oct 26, 2004 CS573: Network Protocols and Standards 1 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 Router Example: Oct 26, 2004 A sends IP Datagram to B A CS573: Network Protocols and Standards B C 2 Routing IP Datagram Indirect delivery (i.e. through intermediate routers) Oct 26, 2004 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 3 Direct and Indirect Routing B wants to send packets to A and C! Host A 204.240.18.10 204.240.18.1 Direct Routing: Packets sent directly using MAC address of A Host B 204.240.18.20 Router Internet 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 C 36.14.0.200 Oct 26, 2004 CS573: Network Protocols and Standards 4 IP Routing Decisions 10.0.0.5 Network 10.0.0.0 20.0.0.6 20.0.0.5 R1 40.0.0.7 30.0.0.6 R3 Network 20.0.0.0 Network 30.0.0.0 R2 30.0.0.7 Network 40.0.0.0 Routing Table of R2 Oct 26, 2004 To Reach Hosts on Network Next Hop Address 20.0.0.0 Direct Delivery 30.0.0.0 Direct Delivery 10.0.0.0 20.0.0.5 40.0.0.0 30.0.0.7 CS573: Network Protocols and Standards 5 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, 2004 CS573: Network Protocols and Standards 6 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, 2004 CS573: Network Protocols and Standards 7 Proxy ARP To Internet Main Router Main Network A B C Router R D E Hidden Network 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 Oct 26, 2004 CS573: Network Protocols and Standards 8 Proxy ARP Advantage of Proxy ARP Router Disadvantages: Can be added without disturbing the routing table in other hosts or routers on that network 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: Oct 26, 2004 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 9 Issues in Addressing A large corporate/campus environment Large number of Local Area Networks If each physical network is assigned a network number: Oct 26, 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 10 Subnetting Solution: Provide the campus with a single class B network Oct 26, 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 11 Subnetting Consider a class B network Oct 26, 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 12 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 26, 2004 H H H CS573: Network Protocols and Standards 13 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 26, 2004 CS573: Network Protocols and Standards 14 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 Internet Part Oct 26, 2004 Local Part Subnet Host CS573: Network Protocols and Standards 15 Subnetting Examples: Class B IP address Internet Part 16bits Oct 26, 2004 Subnet Host 8bits 8bits Internet Part Subnet Host 16bits 3bits 13bits CS573: Network Protocols and Standards 16 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 26, 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 17 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 26, 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 18 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 26, 2004 Subnet mask indicates the network address! CS573: Network Protocols and Standards 19 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 26, 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 20 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 26, 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 21 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 26, 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 22 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 26, 2004 Answer: NO CIDR (Classless Inter Domain Routing) CS573: Network Protocols and Standards 23 Supernet Addressing In the router, the entry consists of: Oct 26, 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 24 IPv6 Motivation Limited address space Support for new applications Oct 26, 2004 Multimedia streams, for example Security Extensibility CS573: Network Protocols and Standards 25 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 26, 2004 CS573: Network Protocols and Standards 26