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Chapter 19 Network Layer: Logical Addressing McGraw-Hill Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Network layer duties McGraw-Hill Figure 19.1 Internetwork Data link layer responsible for data delivery on the network from one node to the next McGraw-Hill Figure 19.3 McGraw-Hill Network layer in an internetwork Brief discussion of switches How to connect multiple devices or to form a large network? Mesh topology Star topology Both are not practical and costly when applied to very large networks because of: Number of links Length of the links Many links will stay idle for most of the time Solution: Switched networks Examples on switching Circuit-switches in telephony network LAN switches in LAN connection, i.e. bridges, and layer-2 switches Packet switches in packet switching networks, i.e. routers McGraw-Hill Figure 8.1 Switched network Switched network: consists of a series of interlinked nodes called switches Switching devices: devices capable of creating a temporary connections between two or more devices linked to the switch and can forward the packet to the next link along the path to its destination. router End system McGraw-Hill Switching McGraw-Hill Note A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into N channels. McGraw-Hill 8-1 CIRCUIT-SWITCHED NETWORKS A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. Each link is normally divided into (N) channels Each connection uses only one dedicated channel on each link. Switch Path is determined during setup phase McGraw-Hill Link divided into 3 channels End system Circuit Switching Data is transmitted as stream of bits, no packetizing is needed In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the termination phase Not efficient for data communication and computer networks because a computer can be connected to another computer through the circuit even if there is no activity for long time. Delay during data transfer is small (no waiting at each switch once the circuit is established) Used in the telephone network McGraw-Hill Figure 8.6 Delay in a circuit-switched network McGraw-Hill Packet switching - Virtual circuit Virtual circuit - Connection-oriented : A connection (handshaking) between the sender and the receiver is established and the complete path for the packets to reach to the destination is determined before transmission of any packets. This path is called virtual circuit or a connection and the address given for each packet is the sequence number of the virtual circuit called Virtual Circuit Identifier (VCI) No physical dedicated path: the path can be used by other virtual circuits Data is packetizied before transmission Packets are guaranteed to arrive in the order they were sent Packets are logically connected to each other, packets travel one after the other The virtual circuit has to be terminated after all packets of a message have been arrived If the virtual circuit router crashes all virtual circuits that go through the router are terminated and paths are lost Used in WAN (Frame relay, ATM) McGraw-Hill Virtual circuit Parameters such as buffer, bandwidth, delay requirements were set in every switch along the path during setup Connect request Connect confirm SW 1 Connect request SW 2 Connect confirm … SW n Connect request Connect confirm Question: why called virtual-circuit? Because resources, e.g., switches, buffers and lines, are shared by packets from multiple connections, not dedicated to one specific connection like in the telephone connection. McGraw-Hill Figure 8.13 Source-to-destination data transfer in a virtual-circuit network McGraw-Hill Figure 8.16 Delay in a virtual-circuit network McGraw-Hill Packet switching - Datagram Datagram - Connectionless service :No handshaking, each packet is sent and routed independently and can follow different paths to reach to the destination. The full address of the source and destination must be attached to each packet. McGraw-Hill No setup delay Packets are not guaranteed to arrive in the order they were sent Robust: If a router crashes only packets inside the router will be lost, other packets can follow other path Used in the Internet Packet switching Note In datagram networks, there is no resource reservation; resources are allocated on demand. McGraw-Hill Figure 8.9 Delay in a datagram network McGraw-Hill Note Switching in the Internet is done by using the datagram approach to packet switching at the network layer. McGraw-Hill Figure 8.8 Routing table in a datagram network McGraw-Hill Note: Communication at the network layer in the Internet is connectionless. McGraw-Hill 19.2 Addressing The address in the network layer of the TCP/IP model is called Internet Address or IP address An IP address is a 32-bit address The IP addresses are unique (each connection has a different address) and universal (must be accepted by any host wants to connect to the internet). Consists of 4 octets (bytes) Network IP addresses are managed by a nonprofit organization called ICANN (International Corporation for Assigned Names and Numbers) to avoid conflicts. McGraw-Hill Assigns addresses to regional Authorities which assign numbers to ISPs Assigns and manages DNS (Domain Name System) The address space of IPv4 is 232 or 4,294,967,296. McGraw-Hill McGraw-Hill Network + Host: Complete IP address Network Address: Host part set to 0 Network ID: identifies the network to which the host is connected Host ID: identifies the interface of the network connection to the host not the host itself McGraw-Hill Figure 19.9 McGraw-Hill Dotted-decimal notation Note: The binary, decimal, and hexadecimal number systems are reviewed in Appendix B. McGraw-Hill Example 1 Change the following IP addresses from binary notation to dotteddecimal notation. a. 10000001 00001011 00001011 11101111 b. 11111001 10011011 11111011 00001111 Solution We replace each group of 8 bits with its equivalent decimal number (see Appendix B) and add dots for separation: a. 129.11.11.239 b. 249.155.251.15 McGraw-Hill Example 2 Change the following IP addresses from dotted-decimal notation to binary notation. a. 111.56.45.78 b. 75.45.34.78 Solution We replace each decimal number with its binary equivalent (see Appendix B): a. b. McGraw-Hill 01101111 00111000 00101101 01001110 01001011 00101101 00100010 01001110 Example Find the error, if any, in the following IP address: 75.45.301.14 Solution In dotted-decimal notation, each number is less than or equal to 255; 301 is outside this range. McGraw-Hill Note: In classful addressing, the address space is divided into five classes: A, B, C, D, and E. McGraw-Hill Finding the classes in binary and dotted-decimal notation McGraw-Hill Figure 19.11 Finding the address class McGraw-Hill Example 3 Find the class of each address: a. 00000001 00001011 00001011 11101111 b. 11110011 10011011 11111011 00001111 Solution See the procedure in Figure 19.11. a. b. McGraw-Hill The first bit is 0; this is a class A address. The first 4 bits are 1s; this is a class E address. Example 4 Find the class of each address: a. 227.12.14.87 b. 252.5.15.111 c. 134.11.78.56 Solution a. b. c. McGraw-Hill The first byte is 227 (between 224 and 239); the class is D. The first byte is 252 (between 240 and 255); the class is E. The first byte is 134 (between 128 and 191); the class is B. Figure 19.13 McGraw-Hill Netid and hostid Classful Addressing Class A Start with binary 0 All 0 reserved (default route) or any network 01111111 (127) reserved for loopback 231 or 2,147,483,648 class A complete IP addresses 27 =128 blocks (network addresses) Number of complete IP addresses in each block is 224=16777216 – (all zeros host - network address, and all ones – broadcast address) Valid Range 1.x.x.x to 126.x.x.x (126 valid blocks) All allocated Class B Start with binary 10 Range 128.x.x.x to 191.x.x.x 230 class B complete IP addresses 214=16384 blocks (network addresses) Number of addresses in each block is 216=65536 – (all zeros host, and all ones) All allocated McGraw-Hill Classful Addressing Class C Start with binary 110 Range 192.x.x.x to 223.x.x.x 229 Class C complete IP addresses 221=2097152 blocks (network addresses) Number of addresses in each block is 256 – (all zeros host, and all ones) class Nearly all allocated Class D Multicast addresses No network/host hierarchy Private addresses Range 10.0.0.0 to 10.255.255.255 224 172.16.0.0 to 172.31.255.255 220 192.168.255.255 216 192.168.0.0 to McGraw-Hill Total Figure 19.14 McGraw-Hill Blocks in class A Note: Millions of class A addresses are wasted. McGraw-Hill Figure 19.15 McGraw-Hill Blocks in class B Note: Many class B addresses are wasted. McGraw-Hill Note: The number of addresses in class C block is smaller than the needs of most organizations. McGraw-Hill Figure 19.16 McGraw-Hill Blocks in class C Figure 19.17 McGraw-Hill Network address Note: In classful addressing, the network address is the one that is assigned to the organization. McGraw-Hill Example 5 Given the address 23.56.7.91, find the network address. Solution The class is A. Only the first byte defines the netid. We can find the network address by replacing the hostid bytes (56.7.91) with 0s. Therefore, the network address is 23.0.0.0. McGraw-Hill Example 6 Given the address 132.6.17.85, find the network address. Solution The class is B. The first 2 bytes defines the netid. We can find the network address by replacing the hostid bytes (17.85) with 0s. Therefore, the network address is 132.6.0.0. McGraw-Hill Example 7 Given the network address 17.0.0.0, find the class. Solution The class is A because the netid is only 1 byte. McGraw-Hill Note: A network address is different from a netid. A network address has both netid and hostid, with 0s for the hostid. McGraw-Hill Figure 19.18 Sample internet Class B Class C Class A McGraw-Hill Note: IP addresses are designed with two levels of hierarchy. McGraw-Hill Table 19.1 Default masks Class In Binary In DottedDecimal Using Slash A 11111111 00000000 00000000 00000000 255.0.0.0 /8 B 11111111 11111111 00000000 00000000 255.255.0.0 /16 C 11111111 111111111 11111111 00000000 255.255.255.0 /24 McGraw-Hill Note: The network address can be found by applying the default mask to any address in the block (including itself). It retains the netid of the block and sets the hostid to 0s. McGraw-Hill Figure 19.19 A network with two levels of hierarchy Addressing without Subnets A class B “Flat Network”, more than 216=65536 hosts How to manage? Performance? Too many hosts on the same LAN (single broadcast domain) will slowdown the LAN performance Solution: Subnetting McGraw-Hill A campus network consisting of LANs for various departments. McGraw-Hill Reduces the routing table entries and size McGraw-Hill Subnetting McGraw-Hill Dividing the network into several smaller groups (subnets) with each group having its own subnet IP address Site looks to rest of internet like single network and routers outside the organization route the packet based on the main Network address Local routers route within subnetted network using subnet address Host portion of address partitioned into subnet number (most significant part) and host number (least significant part) In this case, IP address will have 3 levels (Main network, subnet, host) Subnet mask is a 32-bit consists of zeros and ones that indicates which bits of the IP address are subnet number and which are host number Subnet mask when ANDed with the IP address it gives the subnetwork address Figure 19.23 Class B McGraw-Hill Subnet mask Figure 19.20 A network with three levels of hierarchy (subnetted) McGraw-Hill Routers will use subnet mask 255.255.192.0 or /18 Example 8 A router outside the organization receives a packet with destination address 190.240.7.91 /16. Show how it finds the network address to route the packet. Solution The router follows three steps: 1. The router looks at the first byte of the address to find the class. It is class B. 2. The default mask for class B is 255.255.0.0. or /16 The router ANDs this mask with the address to get 190.240.0.0. 3. The router looks in its routing table to find out how to route the packet to this destination. Later, we will see what happens if this destination does not exist. McGraw-Hill Example 9 A router inside the organization receives the same packet with destination address 190.240.33.91 /19. Show how it finds the subnetwork address to route the packet. Solution The router follows three steps: 1. The router must know the mask. Is 255.255.224.0 or /19 2. The router applies the mask to the address, 190.240.33.91. The subnet address is 190.240.32.0. 3. The router looks in its routing table to find how to route the packet to this destination. Later, we will see what happens if this destination does not exist. McGraw-Hill Address assignment with subnets and subnet routing Assume each subnet has 100 nodes we need only 7 bits for the host, remaining 9 bits of the host part will be assigned to subnets 150.100.0.1 To the rest of the Internet Subnet Mask will be 255.255.255.128 or /25 H1 H2 150.100.12.154 150.100.12.176 150.100.12.128 R = Router Subnet address 150.100.12.129 H = Host R1 150.100.12.4 H3 H4 150.100.12.24 150.100.12.55 150.100.12.0 150.100.12.1 1.A site with class B IP address: 150.100.0.0 R2 H5 2. Outside see all packets to any host within 150.100.15.54 the network is to get the packets to network 150.100.0.0 150.100.15.0 3. Suppose a packet with 150.100.15.11 arrive at R1 from outside: R1 finds the subnet first by doing the following: 150.100.15.11 & 255.255.255.128 = 10010110.01100100.00001111.00001011 & 11111111.11111111.11111111.10000000 = 10010110.01100100.00001111.00000000 i.e., 150.100.15.0 McGraw-Hill 150.100.15.11 Figure 8.7 Obtaining Host IP Address Once a network administrator in an organization obtained a block of addresses from its ISP, it can then assign individual IP addresses to the host and router interfaces It can be done in two ways: Manual configuration: IP address is stored manually by the administrator in a configuration file Solution is using a protocol called Dynamic Host Configuration Protocol (DHCP) McGraw-Hill What about a diskless computer? Or first time booted computer with a disk? What about if the computer has moved from one subnet to another? DHCP is a client-server program 21.1 Dynamic Host Configuration Protocol (DHCP) McGraw-Hill Dynamic Host Configuration Protocol (DHCP) A protocol that provide IP address, subnet mask, IP address of a gateway router, and IP address of DNS server dynamically to a host or to a diskless computer DHCP server keeps two databases (static IP addresses and unused temporary Addresses.) Static IP addresses database maps physical addresses (MAC) to permanent IP addresses (used for diskless workstations) When a host requests an address DHCP will look into the static database first. If no address match is found, DHCP will select the dynamic IP database. DHCP will assign a Temporary Address: selected address from a pool of free addresses and assign it to the host Leasing: DHCP server assigns an IP address for a host for a specific period of time in order not to waste IP addresses After the period expires, host must return the IP address or renew the lease. 21.1 Address Resolution Protocol (ARP) At the network level hosts and routers are recognized by their IP address Packets must pass through physical networks to reach hosts and routers. McGraw-Hill At the physical network, hosts and routers are recognized by their MAC addresses which is local address. ARP is a network layer protocol that translates between Internet IP address and MAC sublayer (layer-2) address Figure 21.4 Four cases using ARP McGraw-Hill Figure 21.1 ARP operation McGraw-Hill Figure 21.3 Encapsulation of ARP packet McGraw-Hill Note An ARP request is broadcast; an ARP reply is unicast. McGraw-Hill Figure 20.5 IPv4 datagram format + Padding McGraw-Hill IPv4 datagram fields Minimum Header length is 20 bytes without options. With options the maximum can go to 60 bytes Largest data that can be carried in the datagram is 65535 – 20 = 65515 Version field: will carry the version number which is 4 = (0100)2 Header length: the length of the header in bytes after dividing it by 4. Min is 20/4 = 5 = (0101)2 and the max is 60/4 = 15 = (1111 )2 Total length: total length of the packet: header + data. Max = 65535 bytes Identification, flags, and offset used for fragmentation and reassembly at the destination. Packet can be fragmented at any node between the source and the destination but reassembly is done ONLY at the destination node. Refer to Figure (20.10) Time to Live is used to prevent lost packets from circulating between routers forever. This field is set to certain value depending on the device operating system. Each router will decrement this field by one and check the value. If the value is zero the packet will be dropped. Protocol: contains a code for what is being carried in the data field. Refer to table (20.4) and Figure (20.8) Header checksum used for checking if there is error in the header only. The checksum is recomputed at each router between the source and the destination. Refer to Figure (20.13) McGraw-Hill Figure 20.8 Protocol field and encapsulated data McGraw-Hill Table 20.4 Protocol values in Hex McGraw-Hill Figure 20.9 Maximum transfer unit (MTU) McGraw-Hill Table 20.5 MTUs for some networks McGraw-Hill Figure 20.13 Example of checksum calculation in IPv4 McGraw-Hill Flags used in fragmentation M=1 means the packet is not the last fragment M=0 means the packet is the last fragment D=1 means Do not fragment the packet McGraw-Hill Figure 20.10 Fragmentation and Reassembly Fragmentation takes place at the sender and routers Reassembly takes place at the receiver ONLY. Fragment at source Reassemble at destination Source IP Router Fragment at router Network McGraw-Hill Destination IP Network Example: Fragmentation Network packet total size (header + data) = 4000 bytes can’t be carried in a single Ethernet frame. Data size = 4000 – 20 = 3980 bytes Divide the data into two packets each has data of size 1480 bytes, and one of size 3980 – 1480 -1480 = 1020 bytes McGraw-Hill Network Address Translation (NAT) – Chapter 19 pages 563 - 566 How To save IP addresses; McGraw-Hill For home users – who are connected to the Internet by dial up, dynamic addresses can be assigned to them for the connection period For business customers and many home users (ADSL), they want to stay connected continuously each user must have its own IP address total number of IP number an ISP can provide will not be enough to cover all customers (for example, class B block can support 65536 only) Solution is using NAT enabled router NAT: enables a company to have large set of unique addresses internally (private addresses) and one address or a small set of addresses externally (public) Figure 19.13 An ISP and NAT ISP has 1000 global IP addresses ONLY ISP has more than 1000 customers McGraw-Hill Private IP addresses Range 10.0.0.0 to 10.255.255.255 224 172.16.0.0 to 172.31.255.255 220 192.168.255.255 216 192.168.0.0 to McGraw-Hill Total Figure 19.10 McGraw-Hill NAT Figure 19.11 Address translation McGraw-Hill Figure 19.12 Translation Source IP 200.24.5.8 McGraw-Hill Network Address Translation (NAT) How does it work? Before the packet leaves the NAT router McGraw-Hill A company is connected to the ISP through a router with NAT software (router is called NAT enabled router). The router has a unique IP address given to the company by the ISP NAT router maintains a translation table that has 65536 entries. Each row has four fields: Private source address, source port number, destination address, NAT port number = NAT table row number Every machine within a company has a unique IP address selected from the set of private addresses usually (10.x.y.z) If a computer inside the company want to connect to a computer outside the network, such as a Web server, the NAT router receives the packet from the computer The NAT router saves the computer's private IP address (source IP) and port number (source port) to the address translation table stored in the router. Then, the router replaces the sending computer's IP address with the router's IP address (global IP address). The router replaces the sending computer's source port with a port number equal to the translation table row number where the router saved the sending computer's address information. Network Address Translation (NAT) When the packet arrives to the NAT router McGraw-Hill When a packet arrives at the NAT router from the ISP router, the NAT router reads the destination port number (NAT given port) on the arriving packet and then uses it in the address translation table to extract the original computer private IP address and original source port number. The destination port and the destination IP are replaced by the original values retrieved from the table.. The packet is then sent to the destination computer. PAT Translation table Private (local) source Address Private (local) source Port NAT Port 172.18.3.1 6789 10000 172.18.3.2 6789 10001 ... ... … McGraw-Hill External (remote) Address 25.8.3.2 25.8.3.2 ... Using PAT allows more than one host to contact same destination NAT Example NAT enabled router Case 1: Host A in the figure sends a packet to IP address 216.109.118.73, port 80, with its local port set to 6798. The resulting entry in the NAT box (assume that the current row index is 9000) is Row # 9000 Case 2: Supposed just after part (A) above, host B sends a packet to the same destination address and port, with it's local port also set to 6798. The resulting entry in the NAT box Row # 9001 McGraw-Hill Advantages of using NAT No need to be allocated range of global addresses from ISP: just one global IP address is used for all devices save IP address can change addresses of devices in local network without notifying outside world can change ISP without changing addresses of devices in local network Can be used as firewall. A computer on an external network cannot connect to your computer unless your computer has initiated the contact. You can browse the Internet and connect to a site, and even download a file; but somebody else cannot use your IP address to connect to a port on your computer. (** Internet routers do not recognize and forward packets with private destination IP addresses) McGraw-Hill