CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 50 pdf

An alternative way to compute the subnet mask and the number of networks is to use the following formulae: The number of usable subnets equals 2 to the power of the assigned subnet bits

To determine the number of bits to be used, the network designer needs to calculate

how many hosts the largest subnetwork requires and the number of subnetworks For

example, assume that this requirement is 30 hosts and five subnetworks To calculate

how many bits to reassign, consult the Usable Hosts row in Table 8-4 For example,

for 30 usable hosts, 3 bits are required This also creates six usable subnetworks, which

satisfies the requirements of this scheme Again, the difference between usable and total

hosts is a result of using the first available address as the ID and the last available

address as the broadcast for each subnetwork Classful routing does not provide the

capability to use these subnetworks, whereas classless routing recovers many of these

“lost” addresses, as shown in Table 8-4 This table illustrates the loss of subnets and

hosts when you don’t use a classless routing protocol

Table 8-3 Subnetting Chart: Subnet Mask Identifier (Two Formats)

Table 8-4 Subnetting Chart: Subnets and Hosts

An alternative way to compute the subnet mask and the number of networks is to use the following formulae:

The number of usable subnets equals 2 to the power of the assigned subnet bits minus 2:

(2power of bits assigned) – 2 = usable subnets For example, 23 – 2 = 6

The number of usable hosts equals 2 to the power of the bits remaining minus 2: (2power of bits remaining) – 2 = usable hosts

For example, 25 – 2 = 30

Creating a Subnet

To create subnets, you must extend the routing portion of the address The Internet

“knows” your network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or 24 routing bits (the network number) The subnet field represents additional routing bits so that the routers within your organization can recognize dif-ferent locations, or subnets, within the whole network

Subnet masks use the same format as IP addresses In other words, each subnet mask

is 32 bits long and is divided into four octets Subnet masks have all 1s in the network and subnetwork portion and all 0s in the host portion By default, if no bits are bor-rowed, the subnet mask for a Class B network is 255.255.0.0 However, if 8 bits were borrowed, the subnet mask for the same Class B network would be 255.255.255.0,

as shown in Figures 8-32 and 8-33 However, because there are two octets in the host field of a Class B network, up to 14 bits can be borrowed to create subnetworks A Class C network has only one octet in the host field Therefore, only up to 6 bits can

be borrowed in Class C networks to create subnetworks

The subnet field always immediately follows the network number That is, the borrowed

bits must be the first n bits of the default host field, where n is the desired size of the new

subnet field, as shown in Figure 8-34 The subnet mask is the tool used by the router to determine which bits are routing bits and which bits are host bits

Determining Subnet Mask Size

Again, subnet masks contain all 1s in the network bit positions (determined by the address class) as well as the subnet bit positions, and they contain all 0s in the remain-ing bit positions, designatremain-ing them as the host portion of an address

Figure 8-32 Network and Host Addresses

Figure 8-33 Binary Conversion Chart

Figure 8-34 Subnetting a Class B Address

IP Address 172 16

Network

Host

Default Subnet Mask

255 255

Network

Host

8-Bit Subnet Mask

255 255

Network

Host Subnet

Use Host Bits, Starting at the High-Order Bit Position

128 64 32 16 8 4 2 1

By default, if you borrow no bits, the subnet mask for a Class B network would be 255.255.0.0, which is the dotted-decimal equivalent of 1s in the 16 bits corresponding

to the Class B network number and 0s in the other 16 bits

If 8 bits were borrowed for the subnet field, the subnet mask would include 8 additional

1 bits and would become 255.255.255.0 For example, if the subnet mask 255.255.255.0 were associated with the Class B address 130.5.2.144 (8 bits borrowed for subnetting), the router would know to route this packet to subnet 130.5.2.0 rather than just to net-work 130.5.0.0, as shown in Figure 8-35

Figure 8-35 Subnet Masking: Class B Address

Another example is the Class C address 197.15.22.131 with a subnet mask of 255.255.255.224 With a value of 224 in the final octet (11100000 in binary), the 24-bit Class C network portion has been extended by 3 bits to make the total 27 bits The

131 in the last octet presents the third usable host address in the subnet 197.15.22.128, as shown in Figure 8-36 The routers in the Internet (that don’t know the subnet mask) only worry about routing to the Class C network 197.15.22.0 The routers inside that network, knowing the subnet mask, look at 27 bits to make a routing decision

Figure 8-36 Subnet Masking: Class C Address

Computing the Subnet Mask and IP Address

Whenever you borrow bits from the host field, it is important to note the number of additional subnets that are being created each time you borrow one more bit You have already learned that you cannot borrow only 1 bit; the fewest you can borrow is 2 Borrowing 2 bits creates four possible subnets (2 × 2) (but you must remember that there are two reserved/unusable subnets) Each time you borrow another bit from the

Network Field Subnetwork

Field

Host Field

11000101 00001111 00010110 10000011

host field, the number of subnets created increases by a power of 2 Eight possible

sub-nets are created by borrowing 3 bits (2 ×2×2) Sixteen possible subnets are created by

borrowing 4 bits (2 ×2×2×2) From these examples and from the binary conversion

chart that was shown in Figure 8-33, it is easy to see that each time you borrow another

bit from the host field, the number of possible subnets doubles

Computing Hosts Per Subnetwork

Each time you borrow 1 bit from a host field, there is 1 less bit remaining that can be

used for host numbers Specifically, each time you borrow another bit from the host

field, the number of host addresses that you can assign decreases by a power of 2 (gets

cut in half)

To understand how this works, consider a Class C network address If there is no

sub-net mask, all 8 bits in the last octet are used for the host field Therefore, 256 (28)

possible addresses are available to assign to hosts (254 usable addresses after you

subtract the two you know you can’t use) Now, imagine that this Class C network

is divided into subnets If you borrow 2 bits from the default 8-bit host field, the host

field decreases in size to 6 bits If you wrote out all the possible combinations of 0s and

1s that could occur in the remaining 6 bits, you would discover that the total number

of possible hosts that could be assigned in each subnet would be reduced to 64 (26)

The number of usable host numbers would be reduced to 62

In the same Class C network, if you borrow 3 bits, the size of the host field decreases

to 5 bits, and the total number of hosts you can assign to each subnet is reduced to

32 (25) The number of usable host numbers decreases to 30

The number of possible host addresses that can be assigned to a subnet is related to

the number of subnets that have been created In a Class C network, for example, if

a subnet mask of 255.255.255.224 has been applied, 3 bits (224 in decimal equals

11100000 in binary) are borrowed from the host field Six usable subnets are created

(8 – 2), each having 30 (32 – 2) usable host addresses

Calculating the Resident Subnetwork Through ANDing

As mentioned earlier, the network or subnet address has all 0s in the host portion To

route a data packet, the router must first determine the destination network/subnet

address To accomplish this, the router performs a logical AND using the destination

host’s IP address and the subnet mask for that network

Imagine that you have a Class B network with the network number 172.16.0.0 After assessing your network’s needs, you decide to borrow 8 bits to create subnets As you learned earlier, when you borrow 8 bits with a Class B network, the subnet mask is 255.255.255.0, as shown in Figure 8-37

Figure 8-37 8 Bits of Subnetting

Someone outside the network sends data to the IP address 172.16.2.120 To determine where to deliver the data, the router ANDs this address with the subnet mask

When the two numbers are ANDed, the host portion of the result is always 0 What

is left is the network number, including the subnet Thus, the data is sent to subnet 172.16.2.0, and only the final router notices that the packet should be delivered to host 120 in that subnet

Now, imagine that you have the same network, 172.16.0.0 This time, however, you decide to borrow only 7 bits for the subnet field The binary subnet mask for this is 11111111.11111111.11111110.00000000 What is this in dotted-decimal notation? Again, someone outside the network sends data to host 172.16.2.120 To determine where to send the data, the router again ANDs this address with the subnet mask As before, when the two numbers are ANDed, the host portion of the result is 0 So what

is different in this second example? Everything looks the same—at least, in decimal The difference is in the number of subnets available and the number of hosts available per subnet You can see this only by comparing the two different subnet masks, as shown in Figure 8-38

Figure 8-38 Network Number Extended by 7 Bits

With 7 bits in the subnet field, there can be only 126 subnets How many hosts can

there be in each subnet? How long is the host field? With 9 bits for host numbers, there

can be 510 hosts in each of those 126 subnets

Summary

In this chapter, you learned the following key points:

■ The differences between, mechanics of, and characteristics of routing and routed

protocols

Lab Activity Basic Subnetting This exercise provides a basic overview of the subnetting and the ANDing processes Given a network address and requirements, you determine the sub-net mask, the number of subsub-nets and hosts per subsub-net, and the number of usable subnets and hosts You also use the ANDing process to determine if a destination IP address is local or remote Finally, you identify valid and invalid

IP host addresses based on a given a network number and subnet mask

Lab Activity Subnetting a Class A Network

In this exercise, you analyze a Class A network address with the number of network bits specified to determine the subnet mask, number of subnets, hosts per subnet, and information about specific subnets

Lab Activity Subnetting a Class B Network

In this exercise, you analyze a Class B network address with the number of net-work bits specified to determine the subnet mask, number of subnets, hosts per subnet, and information about specific subnets

Lab Activity Subnetting a Class C Network

In this exercise, you analyze a Class C network address with the number of network bits specified to determine the subnet mask, number of subnets, hosts per subnet, and information about specific subnets

■ To provide extra flexibility for the network administrator, networks—particu-larly large ones—are often divided into smaller networks called subnetworks or subnets Subnetting allows a network administrator to get around the limitations

of availability of IP addresses by dividing a single network address into many subnets visible only within that single network

■ The function of a subnet mask is to tell devices which part of an address is the network number, including the subnet, and which part is the host

■ Internetworking functions of the network layer include network addressing and best-path selection for data traffic

■ How to explain IP addressing, IP address classes, reserved IP address space, private IP address space, and IP subnetting

To supplement all that you’ve learned in this chapter, refer to the chapter-specific Videos, PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book

Key Terms

algorithm A well-defined rule or process for arriving at a solution to a problem In

networking, algorithms are commonly used to determine the best route for traffic from

a particular source to a particular destination

autonomous system A network or set of networks that are under the administrative

control of a single entity, such as the Cisco.com domain

broadcast A data packet that is sent to all nodes on a network Broadcasts are

identi-fied by a broadcast address

broadcast domain A set of all devices that receive broadcast frames originating from

any device within the set Broadcast domains are typically bounded by routers (or, in a

switched network, by VLANs) because routers do not forward broadcast frames

classless interdomain routing (CIDR) A technique supported by BGP and based on

route aggregation CIDR allows routers to group routes to cut down on the quantity

of routing information carried by the core routers With CIDR, several IP networks

appear to networks outside the group as a single, larger entity

collision domain In Ethernet, the network area within which frames that have collided

are propagated Repeaters and hubs propagate collisions; LAN switches, bridges, and

routers do not

connectionless Data transfer without the existence of a virtual circuit

connection-oriented Data transfer that requires the establishment of a virtual circuit

datagram A logical grouping of information sent as a network layer unit over a

transmission medium without prior establishment of a virtual circuit IP datagrams are

the primary information units in the Internet The terms cell, frame, message, packet,

and segment also describe logical information groupings at various layers of the OSI

reference model and in various technology circles

distance-vector routing A class of routing algorithms that iterate on the number of

hops in a route to find a shortest-path spanning tree Distance-vector routing algorithms

call for each router to send its entire routing table in each update, but only to its

neighbors Distance-vector routing algorithms can be prone to routing loops but are

computationally simpler than link-state routing algorithms Also called the

Bellman-Ford routing algorithm

Exterior Gateway Protocol (EGP) An Internet protocol used to exchange routing

information between autonomous systems Border Gateway Protocol (BGP) is the

most common EGP

hop The passage of a data packet from one network node, typically a router, to another

hop count A routing metric used to measure the distance between a source and a destination RIP uses hop count as its sole metric

Interior Gateway Protocol (IGP) An Internet protocol used to exchange routing information within an autonomous system Examples of common Internet IGPs are IGRP, OSPF, and RIP

Interior Gateway Routing Protocol (IGRP) An IGP developed by Cisco to address the problems associated with routing in large, heterogeneous networks

IP address A 32-bit address assigned to hosts using TCP/IP An IP address belongs

to one of five classes (A, B, C, D, or E) and is written as four octets separated by periods (that is, dotted-decimal format) Each address consists of a network number, an optional subnetwork number, and a host number The network and subnetwork numbers together are used for routing, and the host number is used to address an individual host within the network or subnetwork A subnet mask is used to extract network and subnetwork information from the IP address CIDR provides a new way to represent

IP addresses and subnet masks Also called an Internet address.

MAC address A standardized data link layer address that is required for every device that connects to a LAN Other devices in the network use these addresses to locate specific devices in the network and to create and update routing tables and data struc-tures MAC addresses are 6 bytes long and are controlled by the IEEE Also called a

hardware address, MAC-layer address, or physical address.

NetBIOS Extended User Interface (NetBEUI) An enhanced version of the NetBIOS protocol used by network operating systems such as LAN Manager, LAN Server, Windows for Workgroups, and Windows NT NetBEUI formalizes the transport frame and adds functions NetBEUI implements the OSI LLC2 protocol

octet 8 bits In networking, the term octet is often used (rather than byte) because

some machine architectures employ bytes that are not 8 bits long

packet A logical grouping of information that includes a header containing control information and (usually) user data Packets most often refer to network-layer units of

data The terms datagram, frame, message, and segment also describe logical

informa-tion groupings at various layers of the OSI reference model and in various technology circles