Routed Protocol / IP as a routed protocol

Routed Protocol
Routable and routed protocols
10.1.1 This page will define routed and routable protocols.


A protocol is a set of rules that determines how computers communicate with each other across networks. Computers exchange data messages to communicate with each other. To accept and act on these messages, computers must have sets of rules that determine how a message is interpreted. Examples include messages used to establish a connection to a remote machine, e-mail messages, and files transferred over a network.

A protocol describes the following:

• The required format of a message
• The way that computers must exchange messages for specific activities

A routed protocol allows the router to forward data between nodes on different networks. A routable protocol must provide the ability to assign a network number and a host number to each device. Some protocols, such as IPX, require only a network number. These protocols use the MAC address of the host for the host number. Other protocols, such as IP, require an address with a network portion and a host portion. These protocols also require a network mask to differentiate the two numbers. The network address is obtained by ANDing the address with the network mask.

The reason that a network mask is used is to allow groups of sequential IP addresses to be treated as a single unit. If this grouping were not allowed, each host would have to be mapped individually for routing. This would be impossible, because according to the Internet Software Consortium there are approximately 233,101,500 hosts on the Internet.

The next page will discuss IP.

IP as a routed protocol
10.1.2 This page describes the features and functions of IP.


IP is the most widely used implementation of a hierarchical network-addressing scheme. IP is a connectionless, unreliable, best-effort delivery protocol. The term connectionless means that no dedicated circuit connection is established prior to transmission. IP determines the most efficient route for data based on the routing protocol. The terms unreliable and best-effort do not imply that the system is unreliable and does not work well. They indicate that IP does not verify that data sent on the network reaches its destination. If required, verification is handled by upper layer protocols.

As information flows down the layers of the OSI model, the data is processed at each layer. At the network layer, the data is encapsulated into packets. These packets are also known as datagrams. IP determines the contents of the IP packet header, which includes address information. However, it is not concerned with the actual data. IP accepts whatever data is passed down to it from the upper layers.

The next page examines how a packet travels through a network

Module 10: Routing Fundamentals and Subnets / Overview

Overview
Internet Protocol (IP) is the main routed protocol of the Internet. IP addresses are used to route packets from a source to a destination through the best available path. The propagation of packets, encapsulation changes, and connection-oriented and connectionless protocols are also critical to ensure that data is properly transmitted to its destination. This module will provide an overview for each.


The difference between routing and routed protocols is a common source of confusion. The two words sound similar but are quite different. Routers use routing protocols to build tables that are used to determine the best path to a host on the Internet.

Not all organizations can fit into the three class system of A, B, and C addresses. Flexibility exists within the class system through subnets. Subnets allow network administrators to determine the size of the network they will work with. After they decide how to segment their networks, they can use subnet masks to determine the location of each device on a network.

This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.

Students who complete this module should be able to perform the following tasks:

• Describe routed protocols
• List the steps of data encapsulation in an internetwork as data is routed to Layer 3 devices
• Describe connectionless and connection-oriented delivery
• Name the IP packet fields
• Describe how data is routed
• Compare and contrast different types of routing protocols
• List and describe several metrics used by routing protocols
• List several uses for subnetting
• Determine the subnet mask for a given situation
• Use a subnet mask to determine the subnet ID

Summary of Module 9

Summary
This page summarizes the topics discussed in this module.


The U.S. Department of Defense (DoD) TCP/IP reference model has four layers: the application layer, transport layer, Internet layer, and the network access layer. The application layer handles high-level protocols, issues of representation, encoding, and dialog control. The transport layer provides transport services from the source host to the destination host. The purpose of the Internet layer is to select the best path through the network for packet transmissions. The network access layer is concerned with the physical link to the network media.

Although some layers of the TCP/IP reference model correspond to the seven layers of the OSI model, there are differences. The TCP/IP model combines the presentation and session layer into its application layer. The TCP/IP model combines the OSI data link and physical layers into its network access layer.

Routers use the IP address to move data packets between networks. IP addresses are thirty-two bits long according to the current version IPv4 and are divided into four octets of eight bits each. They operate at the network layer, Layer 3, of the OSI model, which is the Internet layer of the TCP/IP model.

The IP address of a host is a logical address and can be changed. The Media Access Control (MAC) address of the workstation is a 48-bit physical address. This address is usually burned into the network interface card (NIC) and cannot change unless the NIC is replaced. TCP/IP communications within a LAN segment require both a destination IP address and a destination MAC address for delivery. While IP address are unique and routable throughout the Internet, when a packet arrives at the destination network there needs to be a way to automatically map the IP address to a MAC address. The TCP/IP suite has a protocol, called Address Resolution Protocol (ARP), which can automatically obtain MAC addresses for local transmission. A variation on ARP called Proxy ARP will provide the MAC address of an intermediate device for transmission to another network segment.

There are five classes of IP addresses, A through E. Only the first three classes are used commercially. Depending on the class, the network and host part of the address will use a different number of bits. The Class D address is used for multicast groups. Class E addresses are reserved for research use only.

An IP address that has binary zeros in all host bit positions is used to identify the network itself. An address in which all of the host bits are set to one is the broadcast address and is used for broadcasting packets to all the devices on a network.

Public IP addresses are unique. No two machines that connect to a public network can have the same IP address because public IP addresses are global and standardized. Private networks that are not connected to the Internet may use any host addresses, as long as each host within the private network is unique. Three blocks of IP addresses are reserved for private, internal use. These three blocks consist of one Class A, a range of Class B addresses, and a range of Class C addresses. Addresses that fall within these ranges are discarded by routers and not routed on the Internet backbone.

Subnetting is another means of dividing and identifying separate networks throughout the LAN. Subnetting a network means to use the subnet mask to divide the network and break a large network up into smaller, more efficient and manageable segments, or subnets. Subnet addresses include the network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion for the entire network.

A more extendible and scalable version of IP, IP Version 6 (IPv6), has been defined and developed. IPv6 uses 128 bits rather than the 32 bits currently used in IPv4. IPv6 uses hexadecimal numbers to represent the 128 bits. IPv6 is being implemented in select networks and may eventually replace IPv4 as the dominant Internet protocol.

IP addresses are assigned to hosts in the following ways:

• Statically – manually, by a network administrator

• Dynamically – automatically, using reverse address resolution protocol, bootstrap protocol (BOOTP), or Dynamic Host Configuration Protocol (DHCP)

Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP)
9.3.7 This page provides an explanation of how ARP works.


With TCP/IP networking, a data packet must contain both a destination MAC address and a destination IP address. If the packet is missing either one, the data will not pass from Layer 3 to the upper layers. In this way, MAC addresses and IP addresses act as checks and balances for each other. After devices determine the IP addresses of the destination devices, they can add the destination MAC addresses to the data packets.

Some devices will keep tables that contain MAC addresses and IP addresses of other devices that are connected to the same LAN. These are called Address Resolution Protocol (ARP) tables. ARP tables are stored in RAM memory, where the cached information is maintained automatically on each of the devices. It is very unusual for a user to have to make an ARP table entry manually. Each device on a network maintains its own ARP table. When a network device wants to send data across the network, it uses information provided by the ARP table.

When a source determines the IP address for a destination, it then consults the ARP table in order to locate the MAC address for the destination. If the source locates an entry in its table, destination IP address to destination MAC address, it will associate the IP address to the MAC address and then uses it to encapsulate the data. The data packet is then sent out over the networking media to be picked up by the destination device.

There are two ways that devices can gather MAC addresses that they need to add to the encapsulated data. One way is to monitor the traffic that occurs on the local network segment. All stations on an Ethernet network will analyze all traffic to determine if the data is for them. Part of this process is to record the source IP and MAC address of the datagram to an ARP table. So as data is transmitted on the network, the address pairs populate the ARP table. Another way to get an address pair for data transmission is to broadcast an ARP request.

The computer that requires an IP and MAC address pair broadcasts an ARP request. All the other devices on the local area network analyze this request. If one of the local devices matches the IP address of the request, it sends back an ARP reply that contains its IP-MAC pair. If the IP address is for the local area network and the computer does not exist or is turned off, there is no response to the ARP request. In this situation, the source device reports an error. If the request is for a different IP network, there is another process that can be used.

Routers do not forward broadcast packets. If the feature is turned on, a router performs a proxy ARP. Proxy ARP is a variation of the ARP protocol. In this variation, a router sends an ARP response with the MAC address of the interface on which the request was received, to the requesting host. The router responds with the MAC addresses for those requests in which the IP address is not in the range of addresses of the local subnet.

Another method to send data to the address of a device that is on another network segment is to set up a default gateway. The default gateway is a host option where the IP address of the router interface is stored in the network configuration of the host. The source host compares the destination IP address and its own IP address to determine if the two IP addresses are located on the same segment. If the receiving host is not on the same segment, the source host sends the data using the actual IP address of the destination and the MAC address of the router. The MAC address for the router was learned from the ARP table by using the IP address of that router.

If the default gateway on the host or the proxy ARP feature on the router is not configured, no traffic can leave the local area network. One or the other is required to have a connection outside of the local area network.

This page concludes this lesson. The next page will summarize the main points from the module.

DHCP IP address management / Problems in address resolution

DHCP IP address management
9.3.5 This page will explain the features and benefits of DHCP.


Dynamic host configuration protocol (DHCP) is the successor to BOOTP. Unlike BOOTP, DHCP allows a host to obtain an IP address dynamically without the network administrator having to set up an individual profile for each device. All that is required when using DHCP is a defined range of IP addresses on a DHCP server. As hosts come online, they contact the DHCP server and request an address. The DHCP server chooses an address and leases it to that host. With DHCP, the entire network configuration of a computer can be obtained in one message. This includes all of the data supplied by the BOOTP message, plus a leased IP address and a subnet mask.

The major advantage that DHCP has over BOOTP is that it allows users to be mobile. This mobility allows the users to freely change network connections from location to location. It is no longer required to keep a fixed profile for every device attached to the network as was required with the BOOTP system. The importance to this DHCP advancement is its ability to lease an IP address to a device and then reclaim that IP address for another user after the first user releases it. This means that DHCP offers a one to many ratio of IP addresses and that an address is available to anyone who connects to the network. A step-by-step description of the process is shown in Figures through .

The next page describes common problems in address resolution.

Problems in address resolution
9.3.6 This page will discuss address resolution problems.


One of the major problems in networking is how to communicate with other network devices. In TCP/IP communications, a datagram on a local-area network must contain both a destination MAC address and a destination IP address. These addresses must be correct and match the destination MAC and IP addresses of the host device. If it does not match, the datagram will be discarded by the destination host. Communications within a LAN segment require two addresses. There needs to be a way to automatically map IP to MAC addresses. It would be too time consuming for the user to create the maps manually. The TCP/IP suite has a protocol, called Address Resolution Protocol (ARP), which can automatically obtain MAC addresses for local transmission. Different issues are raised when data is sent outside of the local area network.

Communications between two LAN segments have an additional task. Both the IP and MAC addresses are needed for both the destination host and the intermediate routing device. TCP/IP has a variation on ARP called Proxy ARP that will provide the MAC address of an intermediate device for transmission outside the LAN to another network segment.

The next page will describe Address Resolution Protocol (ARP).

BOOTP IP address assignment

BOOTP IP address assignment
9.3.4 This page will introduce BOOTP.


The bootstrap protocol (BOOTP) operates in a client-server environment and only requires a single packet exchange to obtain IP information. However, unlike RARP, BOOTP packets can include the IP address, as well as the address of a router, the address of a server, and vendor-specific information.

One problem with BOOTP, however, is that it was not designed to provide dynamic address assignment. With BOOTP, a network administrator creates a configuration file that specifies the parameters for each device. The administrator must add hosts and maintain the BOOTP database. Even though the addresses are dynamically assigned, there is still a one to one relationship between the number of IP addresses and the number of hosts. This means that for every host on the network there must be a BOOTP profile with an IP address assignment in it. No two profiles can have the same IP address. Those profiles might be used at the same time and that would mean that two hosts have the same IP address.

A device uses BOOTP to obtain an IP address when starting up. BOOTP uses UDP to carry messages. The UDP message is encapsulated in an IP packet. A computer uses BOOTP to send a broadcast IP packet using a destination IP address of all 1s, 255.255.255.255 in dotted decimal notation. A BOOTP server receives the broadcast and then sends back a broadcast. The client receives a frame and checks the MAC address. If the client finds its own MAC address in the destination address field and a broadcast in the IP destination field, it takes and stores the IP address and other information supplied in the BOOTP reply message. A step-by-step description of the process is shown in Figures through .

The next page will discuss Dynamic Host Configuration Protocol (DHCP).

RARP IP address assignment

RARP IP address assignment
9.3.3 This page will discuss RARP address assignment.


Reverse Address Resolution Protocol (RARP) associates a known MAC addresses with an IP addresses. This association allows network devices to encapsulate data before sending the data out on the network. A network device, such as a diskless workstation, might know its MAC address but not its IP address. RARP allows the device to make a request to learn its IP address. Devices using RARP require that a RARP server be present on the network to answer RARP requests.

Consider an example where a source device wants to send data to another device. In this example, the source device knows its own MAC address but is unable to locate its own IP address in the ARP table. The source device must include both its MAC address and IP address in order for the destination device to retrieve data, pass it to higher layers of the OSI model, and respond to the originating device. Therefore, the source initiates a process called a RARP request. This request helps the source device detect its own IP address. RARP requests are broadcast onto the LAN and are responded to by the RARP server which is usually a router.

RARP uses the same packet format as ARP. However, in a RARP request, the MAC headers and operation code are different from an ARP request. The RARP packet format contains places for MAC addresses of both the destination and source devices. The source IP address field is empty. The broadcast goes to all devices on the network. Figures , , and depict the destination MAC address as FF:FF:FF:FF:FF:FF. Workstations running RARP have codes in ROM that direct them to start the RARP process. A step-by-step layout of the RARP process is illustrated in Figures through .

The next page will discuss the Bootstrap Protocol (BOOTP).