Due to the number of hardware possibilities for a network, there must be a set of rules for how data should be transmitted across the connection media. A protocol defines how the network devices and computers talk to one another. What protocol does Phoenix Elementary School District # 1 use? TCP/IP - Transmission Control Protocol / Internet Protocol Each time a network device manipulates data, it obeys TCP/IP rules. When data packets are sent, they may not arrive at their destination in order. Example: E-mail messages are split into packets. If the data gets spread over several packets, TCP/IP puts it back together so that the message makes sense. IP Internet Protocol, is responsible for basic network connectivity. IP provides a place to send and receive data - a network address. Network addresses, unlike physical addresses, are not burned into any hardware. Network addresses are assigned by network administrators and are logically configured into network devices. Every computer or device on a TCP/IP network must have a numeric address. The IP on any computer understands how and where to send messages to these addresses. TCP Transmission Control Protocol uses IP to deliver packets to the upper-layer applications and provides a reliable stream of data among computers on the network. Error checking and sequence numbering are two of TCP s more important functions. Once a packet arrives at the correct IP address, TCP examines the data. On both the sending and receiving computers, it establishes a dialog to communicate about the data that is being transmitted. TCP is connection oriented because it tells the network to resend lost data. TCP/IP works on 3 types of services: 1) Connection services: Determine how data gets from one computer onto the network cable (or other connection medium) and how that data moves from the network cable to the next computer. The connection services do not guarantee that the data will arrive at the destination in the right order or that it will arrive at all. 2) Transport services: Provide completely reliable communication between computers. The packets are numbered to make sure the data can be placed in the right order even when the packets arrive out of sequence. The computers then perform error checking to make sure that no packets are lost or damaged. 6
TCP/IP works on 3 types of services (cont.): 3) Application services: Let an application on one computer talk to a similar application on another computer in order to perform tasks such as copying files. These services depend on connection services and transport services for reliable and efficient communication. TCP/IP refers to each local network as a subnet. IP uses addresses to identify (subnets which are network numbers) and (devices, such as computers, which are called hosts.) The IP addresses are 32 bit numbers that contain both a subnet address and a host address. IP addresses are divided into four octets (8-bit sections) called dotted-decimal notation. Each device must have a unique IP address. Example of Dotted Decimal Notation and the corresponding Binary Notation: 170.203.93.5 10101010 11001011 1011101 00000101 TCP/IP protocols are divided into three categories: Class A - a small number of networks that have a large number of hosts. The first octet has a value of 0 through 127. There are 126 Class A addresses, each of which can support 16,777,216 hosts. Class B - networks with an intermediate number of hosts. The first octet has a value of 128 through 191. There are 16,384 Class B addresses, each of which can support 65,536 hosts. Class C - a large number of networks that have a small number of hosts. The first octet has a value of 192 through 223. There are 2,097,152 Class C addresses, each of which can support 254 hosts. 7
Each IP address consists of two fields: A netid field that is the logical network address of the subnet to which the computer is attached. A hostid field, which is the logical device address that uniquely identifies each host on a subnet. Together, the netid and the hostid provide each host on an internetwork with a unique IP address. Class A NNNNNNNN HHHHHHHH HHHHHHHH HHHHHHHH Class B NNNNNNNN NNNNNNNN HHHHHHHH HHHHHHHH Class C NNNNNNNN NNNNNNNN NNNNNNNN HHHHHHHH N = Netid H = Hostid Obtaining Internet Addresses If a network is isolated and not connected to the Internet, any addresses can be used; however, on the Internet, the addresses must be assigned by the Internet Network Information Center (InterNIC). Internet addresses are getting scarce, and the ability to obtain enough addresses to assign becomes more difficult as the Internet grows. Two alternatives are used to limit the number of IP address ranges required by users of the Internet. They are: Alternative 1: Subnets Subnets divide one network into multiple smaller networks. The separate networks are normally interconnected by network devices called routers. Not every environment requires subnets. If an organization s class C network has 254 or fewer hosts and the network is entirely in one building, there s no reason to subnet it. However, if the organization s network expands into multiple locations, the network administrator can split the existing network into pieces, one piece for each location. Every host on a network must be configured with the same subnet ID. Like IP addresses, subnet ID s can be represented in dotted decimal notation. Subnet ID s or masks make it easier and faster for IP to identify the netid portion of the IP address. Also, they make it possible to suballocate network addresses. Both the IP address and the subnet mask are examined in their binary format when subneting is evaluated. Boolean arithmetic called ANDing is used during this process. An explanation of that is beyond the scope of this document. 8
The default subnet mask for the three classes of networks are displayed below: Class A: 255.0.0.0 11111111 00000000 00000000 00000000 Class B: 255.255.0.0 11111111 11111111 00000000 00000000 Class C: 255.255.255.0 11111111 11111111 11111111 00000000 A Zero [ 0 ] in a subnet mask indicates that the corresponding bit in the IP address is part of the host ID. Routing Packets to Remote Subnets When hosts are connected to an internetwork, each host is configured with the IP address of a default gateway or router. If IP determines that the source and destination subnet addresses don t match, IP addresses the packet to the default router. IP determines where to route packets by consulting routing tables contained within the routers. Alternative 2: Private IP Addresses Packets of information whose source or destination is in a private address space are dropped by routers, and information about private address spaces is not passed between routers. IP addresses specified in RFC 1918, Address Allocation for Private Internets, define the IP address ranges that can be used and are listed below. Class A Type: 10.0.0.0 Class B Type: 172.16.0.0 to 172.31.0.0 Class C Type: 192.168.0.0 to 192.168.255.0 If a network that uses private addresses needs to be connected to the Internet, it may do one of two things: renumber all of its computers to use IP numbers assigned by InterNIC, or use a multihomed device as a connection to the Internet. 9
A multihomed device has multiple IP addresses. At least one address needs to be on the private network, and one (or more) of its addresses needs to be on the public network. An application typically a firewall or proxy server - must be set up on the multihomed device to process requests from the private network, and act as a broker between the private network and external resources the Internet. DHCP Dynamic Host Configuration Protocol Provides automatic configuration of IP addresses. All addresses may be allocated from a range of IP addresses available for lease. This range of addresses is called a scope. This takes place when a computer demands TCP/IP access to the network. DHCP addresses are leased for a specific duration. When the lease approaches expiration, an active client negotiates to renew the lease. If the current address cannot be reassigned, a new address is assigned to the client. Addresses that are not renewed are returned to the address pool. Phoenix Elementary School District # 1 s server FS_DO1 is a DHCP server. Each site / location s file server acts as a DHCP server for that site. The number assigned by the site server is then authenticated by the main DHCP server at the district office. DNS Domain Name Service As previously stated, any computer that needs to be visible on the Internet must have a unique IP address. It also has a name that conforms with its network s naming convention. DNS was designed to map Internet addresses, such as 12.7.70.178, to names, such as host.subnet.company.com. The name parts proceed from most specific to least specific, as you move from the first to last parts of the name. The first name is often the name of the computer. The next part is generally the subnet the computer is on. The second-to-last name part is often the company s name, and the last name part describes the kind of company it is. Without DNS you would be required to access information from remote systems using the IP address instead of a valid computer name. Names are much easier to remember than IP addresses; consequently, DNS is used to resolve a computer name into the appropriate TCP/IP address - www.phxelem.k12.az.gov is much easier to remember that 207.105.154.194. DNS is a distributed database. On the Internet, the root of this database is written as a single dot (. ). This root database is maintained by InterNIC. The root of DNS is then sub-divided into toplevel entries such as [.com ] for commercial organizations, [.gov ] for government institutions, [.net ] for Internet Service Providers, and [.edu ] for educational institutions. There are others that are used for other purposes, such as those used to specify other countries. A server that can tell you an IP address if you give it a domain name is called a domain name server. DNS clients ask these servers to return IP addresses for specified domain names. In completing its tasks, one DNS may ask another DNS for help, an so on, until an answer is found and a reply is sent to the DNS client. 10