Classes of network IP addresses

Classes of network IP addresses
10.3.1 This page will review the classes of IP addresses. The combined classes of IP addresses offer a range from 256 to 16.8 million hosts.


To efficiently manage a limited supply of IP addresses, all classes can be subdivided into smaller subnetworks. Figure provides an overview of the division between networks and hosts.

The next page will explain why subnetting is important

Introduction to and reason for subnetting
10.3.2 This page will describe how subnetting works and why it is important.


To create the subnetwork structure, host bits must be reassigned as network bits. This is often referred to as ‘borrowing’ bits. However, a more accurate term would be ‘lending’ bits. The starting point for this process is always the leftmost host bit, the one closest to the last network octet.

Subnet addresses include the Class A, Class B, and Class C network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion of the major IP address. This is done by re-assigning bits from the host portion to the original network portion of the address. - The ability to divide the original host portion of the address into the new subnet and host fields provides addressing flexibility for the network administrator.

In addition to the need for manageability, subnetting enables the network administrator to provide broadcast containment and low-level security on the LAN. Subnetting provides some security since access to other subnets is only available through the services of a router. Further, access security may be provided through the use of access lists. These lists can permit or deny access to a subnet, based on a variety of criteria, thereby providing more security. Access lists will be studied later in the curriculum. Some owners of Class A and B networks have also discovered that subnetting creates a revenue source for the organization through the leasing or sale of previously unused IP addresses.

Subnetting is an internal function of a network. From the outside, a LAN is seen as a single network with no details of the internal network structure. This view of the network keeps the routing tables small and efficient. Given a local node address of 147.10.43.14 on subnet 147.10.43.0, the world outside the LAN sees only the advertised major network number of 147.10.0.0. The reason for this is that the local subnet address of 147.10.43.0 is only valid within the LAN where subnetting is applied.

The next page will discuss subnet masks.

Routing protocols

Routing Protocols
10.2.9 This page will describe different types of router protocols.
RIP is a distance vector routing protocol that uses hop count as its metric to determine the direction and distance to any link in the internetwork. If there are multiple paths to a destination, RIP selects the path with the least number of hops. However, because hop count is the only routing metric used by RIP, it does not always select the fastest path to a destination. Also, RIP cannot route a packet beyond 15 hops. RIP Version 1 (RIPv1) requires that all devices in the network use the same subnet mask, because it does not include subnet mask information in routing updates. This is also known as classful routing.
RIP Version 2 (RIPv2) provides prefix routing, and does send subnet mask information in routing updates. This is also known as classless routing. With classless routing protocols, different subnets within the same network can have different subnet masks. The use of different subnet masks within the same network is referred to as variable-length subnet masking (VLSM).
IGRP is a distance-vector routing protocol developed by Cisco. IGRP was developed specifically to address problems associated with routing in large networks that were beyond the range of protocols such as RIP. IGRP can select the fastest available path based on delay, bandwidth, load, and reliability. IGRP also has a much higher maximum hop count limit than RIP. IGRP uses only classful routing.
OSPF is a link-state routing protocol developed by the Internet Engineering Task Force (IETF) in 1988. OSPF was written to address the needs of large, scalable internetworks that RIP could not.
Intermediate System-to-Intermediate System (IS-IS) is a link-state routing protocol used for routed protocols other than IP. Integrated IS-IS is an expanded implementation of IS-IS that supports multiple routed protocols including IP.
Like IGRP, EIGRP is a proprietary Cisco protocol. EIGRP is an advanced version of IGRP. Specifically, EIGRP provides superior operating efficiency such as fast convergence and low overhead bandwidth. EIGRP is an advanced distance-vector protocol that also uses some link-state protocol functions. Therefore, EIGRP is sometimes categorized as a hybrid routing protocol.
Border Gateway Protocol (BGP) is an example of an External Gateway Protocol (EGP). BGP exchanges routing information between autonomous systems while guaranteeing loop-free path selection. BGP is the principal route advertising protocol used by major companies and ISPs on the Internet. BGP4 is the first version of BGP that supports classless interdomain routing (CIDR) and route aggregation. Unlike common Internal Gateway Protocols (IGPs), such as RIP, OSPF, and EIGRP, BGP does not use metrics like hop count, bandwidth, or delay. Instead, BGP makes routing decisions based on network policies, or rules using various BGP path attributes.
The Lab Activity will help students understand the price of a small router.
This page concludes this lesson. The next lesson will focus on the mechanics of subnetting. The first page covers the different classes of IP addresses.

IGP and EGP / Link state and distance vector

IGP and EGP
10.2.7 This page will introduce two types of routing protocols.


An autonomous system is a network or set of networks under common administrative control, such as the cisco.com domain. An autonomous system consists of routers that present a consistent view of routing to the external world.

Two families of routing protocols are Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs).

IGPs route data within an autonomous system:

• RIP and RIPv2
• IGRP
• EIGRP
• OSPF
• Intermediate System-to-Intermediate System (IS-IS) protocol

EGPs route data between autonomous systems. An example of an EGP is BGP.

The next page will define link-state and distance vector protocols.

Link state and distance vector
10.2.8 Routing protocols can be classified as either IGPs or EGPs. Which type is used depends on whether a group of routers is under a single administration or not. IGPs can be further categorized as either distance-vector or link-state protocols. This page describes distance-vector and link-state routing and explains when each type of routing protocol is used.


The distance-vector routing approach determines the distance and direction, vector, to any link in the internetwork. The distance may be the hop count to the link. Routers using distance-vector algorithms send all or part of their routing table entries to adjacent routers on a periodic basis. This happens even if there are no changes in the network. By receiving a routing update, a router can verify all the known routes and make changes to its routing table. This process is also known as “routing by rumor”. The understanding that a router has of the network is based upon the perspective of the adjacent router of the network topology.

Examples of distance-vector protocols include the following:

• Routing Information Protocol (RIP) – The most common IGP in the Internet, RIP uses hop count as its only routing metric.

• Interior Gateway Routing Protocol (IGRP) – This IGP was developed by Cisco to address issues associated with routing in large, heterogeneous networks.

• Enhanced IGRP (EIGRP) – This Cisco-proprietary IGP includes many of the features of a link-state routing protocol. Because of this, it has been called a balanced-hybrid protocol, but it is really an advanced distance-vector routing protocol.

Link-state routing protocols were designed to overcome limitations of distance vector routing protocols. Link-state routing protocols respond quickly to network changes sending trigger updates only when a network change has occurred. Link-state routing protocols send periodic updates, known as link-state refreshes, at longer time intervals, such as every 30 minutes.

When a route or link changes, the device that detected the change creates a link-state advertisement (LSA) concerning that link. The LSA is then transmitted to all neighboring devices. Each routing device takes a copy of the LSA, updates its link-state database, and forwards the LSA to all neighboring devices. This flooding of LSAs is required to ensure that all routing devices create databases that accurately reflect the network topology before updating their routing tables.

Link-state algorithms typically use their databases to create routing table entries that prefer the shortest path. Examples of link-state protocols include Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS).

The Interactive Media Activity will identify the differences between link-state and distance vector routing protocols.

The next page will discuss routing protocols.

Routing tables / Routing algorithms and metrics

Routing tables
10.2.5 This page will describe the functions of a routing table.


Routers use routing protocols to build and maintain routing tables that contain route information. This aids in the process of path determination. Routing protocols fill routing tables with a variety of route information. This information varies based on the routing protocol used. Routing tables contain the information necessary to forward data packets across connected networks. Layer 3 devices interconnect broadcast domains or LANs. A hierarchical address scheme is required for data transfers.

Routers keep track of the following information in their routing tables:

• Protocol type – Identifies the type of routing protocol that created each entry.

• Next-hop associations – Tell a router that a destination is either directly connected to the router or that it can be reached through another router called the next-hop on the way to the destination. When a router receives a packet, it checks the destination address and attempts to match this address with a routing table entry.

• Routing metric – Different routing protocols use different routing metrics. Routing metrics are used to determine the desirability of a route. For example, RIP uses hop count as its only routing metric. IGRP uses bandwidth, load, delay, and reliability metrics to create a composite metric value.

• Outbound interfaces – The interface that the data must be sent out of to reach the final destination.

Routers communicate with one another to maintain their routing tables through the transmission of routing update messages. Some routing protocols transmit update messages periodically. Other protocols send them only when there are changes in the network topology. Some protocols transmit the entire routing table in each update message and some transmit only routes that have changed. Routers analyze the routing updates from directly-connected routers to build and maintain their routing tables.

The next page will explain routing algorithms and metrics.

Routing algorithms and metrics
10.2.6 This page will define algorithms and metrics as they relate to routers.


An algorithm is a detailed solution to a problem. Different routing protocols use different algorithms to choose the port to which a packet should be sent. Routing algorithms depend on metrics to make these decisions.

Routing protocols often have one or more of the following design goals:

• Optimization – This is the capability of a routing algorithm to select the best route. The route will depend on the metrics and metric weights used in the calculation. For example, one algorithm may use both hop count and delay metrics, but may consider delay metrics as more important in the calculation.

• Simplicity and low overhead – The simpler the algorithm, the more efficiently it will be processed by the CPU and memory in the router. This is important so that the network can scale to large proportions, such as the Internet.

• Robustness and stability – A routing algorithm should perform correctly when confronted by unusual or unforeseen circumstances, such as hardware failures, high load conditions, and implementation errors.

• Flexibility – A routing algorithm should quickly adapt to a variety of network changes. These changes include router availability, router memory, changes in bandwidth, and network delay.

• Rapid convergence – Convergence is the process of agreement by all routers on available routes. When a network event causes changes in router availability, updates are needed to reestablish network connectivity. Routing algorithms that converge slowly can cause data to be undeliverable.

Routing algorithms use different metrics to determine the best route. Each routing algorithm interprets what is best in its own way. A routing algorithm generates a number called a metric value for each path through a network. Sophisticated routing algorithms base route selection on multiple metrics that are combined in a composite metric value.
Typically, smaller metric values indicate preferred paths.


Metrics can be based on a single characteristic of a path, or can be calculated based on several characteristics. The following metrics are most commonly used by routing protocols:

• Bandwidth – Bandwidth is the data capacity of a link. Normally, a 10-Mbps Ethernet link is preferable to a 64-kbps leased line.

• Delay – Delay is the length of time required to move a packet along each link from a source to a destination. Delay depends on the bandwidth of intermediate links, the amount of data that can be temporarily stored at each router, network congestion, and physical distance.

• Load – Load is the amount of activity on a network resource such as a router or a link.

• Reliability – Reliability is usually a reference to the error rate of each network link.

• Hop count – Hop count is the number of routers that a packet must travel through before reaching its destination. Each router is equal to one hop. A hop count of four indicates that data would have to pass through four routers to reach its destination. If multiple paths are available to a destination, the path with the least number of hops is preferred.

• Ticks – The delay on a data link using IBM PC clock ticks. One tick is approximately 1/18 second.

• Cost – Cost is an arbitrary value, usually based on bandwidth, monetary expense, or other measurement, that is assigned by a network administrator.

The next page will discuss two types of routing protocols.

Routed versus routing / Path determination

Routed versus routing
10.2.3 This page explains the differences between routing protocols and routed protocols.


Routed or routable protocols are used at the network layer to transfer data from one host to another across a router. Routed protocols transport data across a network. Routing protocols allow routers to choose the best path for data from a source to a destination.

Some functions of a routed protocol are as follows:

• Includes any network protocol suite that provides enough information in its network layer address to allow a router to forward it to the next device and ultimately to its destination

• Defines the format and use of the fields within a packet

The Internet Protocol (IP) and Novell Internetwork Packet Exchange (IPX) are examples of routed protocols. Other examples include DECnet, AppleTalk, Banyan VINES, and Xerox Network Systems (XNS).

Routers use routing protocols to exchange routing tables and share routing information. In other words, routing protocols enable routers to route routed protocols.

Some functions of a routing protocol are as follows:

• Provides processes used to share route information

• Allows routers to communicate with other routers to update and maintain the routing tables

Examples of routing protocols that support the IP routed protocol include RIP, IGRP, OSPF, BGP, and EIGRP.

Path determination
10.2.4 This page will explain how path determination occurs.


Path determination occurs at the network layer. A router uses path determination to compare a destination address to the available routes in its routing table and select the best path. The routers learn of these available routes through static routing or dynamic routing. Routes configured manually by the network administrator are static routes. Routes learned by others routers using a routing protocol are dynamic routes.

The router uses path determination to decide which port to send a packet out of to reach its destination. This process is also referred to as routing the packet. Each router that the packet encounters along the way is called a hop. The hop count is the distanced traveled. Path determination can be compared to a person who drives from one location in a city to another. The driver has a map that shows which streets lead to the destination, just as a router has a routing table. The driver travels from one intersection to another just as a packet travels from one router to another in each hop. At any intersection, the driver can choose to turn left, turn right, or go straight ahead. This is similar to how a router chooses the outbound port through which a packet is sent.

The decisions of a driver are influenced by factors such as traffic, the speed limit, the number of lanes, tolls, and whether or not a road is frequently closed. Sometimes it is faster to take a longer route on a smaller, less crowded back street instead of a highway with a lot of traffic. Similarly, routers can make decisions based on the load, bandwidth, delay, cost, and reliability of a network link.

The following process is used to determine the path for every packet that is routed:

• The router compares the IP address of the packet that it received to the IP tables that it has.
• The destination address is obtained from the packet.
• The mask of the first entry in the routing table is applied to the destination address.
• The masked destination and the routing table entry are compared.
• If there is a match, the packet is forwarded to the port that is associated with that table entry.
• If there is not a match, the next entry in the table is checked.
• If the packet does not match any entries in the table, the router checks to see if a default route has been set.
• If a default route has been set, the packet is forwarded to the associated port. A default route is a route that is configured by the network administrator as the route to use if there are no matches in the routing table.
• If there is no default route, the packet is discarded. A message is often sent back to the device that sent the data to indicate that the destination was unreachable.

The next page will explain how routing protocols build and maintain routing tables.

Routing versus switching

Routing versus switching
10.2.2 This page will compare and contrast routing and switching. Routers and switches may seem to perform the same function. The primary difference is that switches operate at Layer 2 of the OSI model and routers operate at Layer 3. This distinction indicates that routers and switches use different information to send data from a source to a destination.


The relationship between switching and routing can be compared to local and long-distance telephone calls. When a telephone call is made to a number within the same area code, a local switch handles the call. The local switch can only keep track of its local numbers. The local switch cannot handle all the telephone numbers in the world. When the switch receives a request for a call outside of its area code, it switches the call to a higher-level switch that recognizes area codes. The higher-level switch then switches the call so that it eventually gets to the local switch for the area code dialed.

The router performs a function similar to that of the higher-level switch in the telephone example. Figure shows the ARP tables for Layer 2 MAC addresses and routing tables for Layer 3 IP addresses. Each computer and router interface maintains an ARP table for Layer 2 communication. The ARP table is only effective for the broadcast domain to which it is connected. The router also maintains a routing table that allows it to route data outside of the broadcast domain. Each ARP table entry contains an IP-MAC address pair.

The Layer 2 switch builds its forwarding table using MAC addresses. When a host has data for a non-local IP address, it sends the frame to the closest router. This router is also known as its default gateway. The host uses the MAC address of the router as the destination MAC address.

A switch interconnects segments that belong to the same logical network or subnetwork. For non-local hosts, the switch forwards the frame to the router based on the destination MAC address. The router examines the Layer 3 destination address of the packet to make the forwarding decision. Host X knows the IP address of the router because the IP configuration of the host contains the IP address of the default gateway.

Just as a switch keeps a table of known MAC addresses, the router keeps a table of IP addresses known as a routing table. MAC addresses are not logically organized. IP addresses are organized in a hierarchy. A switch can handle a limited number of unorganized MAC addresses since it only has to search its table for addresses within its segment. Routers require an organized address system that can group similar addresses together and treat them as a single network unit until the data reaches the destination segment.

If IP addresses were not organized, the Internet would not work. This could be compared to a library that contained millions of individual pages of printed material in a large pile. This material is useless because it is impossible to locate an individual document. If the pages are identified and organized into books and each book is listed in a book index, it will be a lot easier to locate and use the data.

Another difference between switched and routed networks is switched networks do not block broadcasts. As a result, switches can be overwhelmed by broadcast storms. Routers block LAN broadcasts, so a broadcast storm only affects the broadcast domain from which it originated. Since routers block broadcasts, they also provide a higher level of security and bandwidth control than switches.

The next page will compare routing and routed protocols.

Routing overview

Routing overview
10.2.1 This page will discuss routing and the two main functions of a router.


Routing is an OSI Layer 3 function. Routing is a hierarchical organizational scheme that allows individual addresses to be grouped together. These individual addresses are treated as a single unit until the destination address is needed for final delivery of the data. Routing finds the most efficient path from one device to another. The primary device that performs the routing process is the router.

The following are the two key functions of a router:

• Routers must maintain routing tables and make sure other routers know of changes in the network topology. They use routing protocols to communicate network information with other routers.

• When packets arrive at an interface, the router must use the routing table to determine where to send them. The router switches the packets to the appropriate interface, adds the frame information for the interface, and then transmits the frame.

A router is a network layer device that uses one or more routing metrics to determine the optimal path along which network traffic should be forwarded. Routing metrics are values that are used to determine the advantage of one route over another. Routing protocols use various combinations of metrics to determine the best path for data.

Routers interconnect network segments or entire networks. Routers pass data frames between networks based on Layer 3 information. Routers make logical decisions about the best path for the delivery of data. Routers then direct packets to the appropriate output port to be encapsulated for transmission. Stages of the encapsulation and de-encapsulation process occur each time a packet transfers through a router. The router must de-encapsulate the Layer 2 data frame to access and examine the Layer 3 address. As shown in Figure , the complete process of sending data from one device to another involves encapsulation and de-encapsulation on all seven OSI layers. The encapsulation process breaks up the data stream into segments, adds the appropriate headers and trailers, and then transmits the data. The de-encapsulation process removes the headers and trailers and then recombines the data into a seamless stream.

This course focuses on the most common routable protocol, which is IP. Other examples of routable protocols include IPX/SPX and AppleTalk. These protocols provide Layer 3 support. Non-routable protocols do not provide Layer 3 support. The most common non-routable protocol is NetBEUI. NetBEUI is a small, fast, and efficient protocol that is limited to frame delivery within one segment.

The next page will compare routing and switching.