This means that the only Class C subnet masks can be the following:
Binary Decimal CIDR
---------------------------------------------------------
10000000 = 128 /25
11000000 = 192 /26
11100000 = 224 /27
11110000 = 240 /28
11111000 = 248 /29
11111100 = 252 /30
When you’ve chosen a possible subnet mask for your network and need to determine the number of subnets, valid hosts, and broadcast addresses of a subnet that the mask provides, all you need to do is answer five simple questions:
How many subnets does the chosen subnet mask produce?
How many valid hosts per subnet are available?
What are the valid subnets?
What’s the broadcast address of each subnet?
What are the valid hosts in each subnet?
By the way, do you notice anything interesting about that list of subnet values—a pattern, maybe? Ah ha! That’s exactly why I had you memorize the binary-to-decimal numbers at the beginning of this section. Since subnet mask bits start on the left, move to the right, and can’t skip bits, the numbers are always the same regardless of the class of address. Memorize this pattern.
There are loads of reasons in favor of subnetting, including the following benefits:
Reduced network traffic We all appreciate less traffic of any kind. Networks are no different. Without trusty routers, packet traffic could grind the entire network down to a near standstill. With routers, most traffic will stay on the local network; only packets destined for other networks will pass through the router. Routers create broadcast domains. The more broadcast domains you create, the smaller the broadcast domains and the less network traffic on each network segment.
Optimized network performance This is a result of reduced network traffic.
Simplified management It’s easier to identify and isolate network problems in a group of smaller connected networks than within one gigantic network.
Facilitated spanning of large geographical distances Because WAN links are considerably slower and more expensive than LAN links, a single large network that spans long distances can create problems in every area previously listed. Connecting multiple smaller networks makes the system more efficient.
It is important to remember that when studying for your Cisco exams, make sure you read very carefully and understand if Cisco is asking you not to use subnet-zero. There are instances where this may happen.
Before we move on to designing and creating a subnet mask, you need to understand that in this first section we will be discussing classful routing, which means that all hosts (all nodes) in the network use the exact same subnet mask. When we move on to Variable Length Subnet Masks (VLSMs), I’ll discuss classless routing, which means that each network segment can use a different subnet mask.
Follow these steps:
Determine the number of required network IDs:
One for each subnet
One for each wide area network connection
Determine the number of required host IDs per subnet:
One for each TCP/IP host
One for each router interface
Based on the above requirements, create the following:
One subnet mask for your entire network
A unique subnet ID for each physical segment
A range of host IDs for each subnet
Here’s a list of powers of 2 that you should commit to memory:
21 = 2
22 = 4
23 = 8
24 = 16
25 = 32
26 = 64
27 = 128
28 = 256
29 = 512
210 = 1,024
211 = 2,048
212 = 4,096
213 = 8,192
214 = 16,384
For example, all you have to do to remember the value of 29 is to first know that 28 = 256. Why? Because when you double 2 to the eighth power (256), you get 29 (or 512). To determine the value of 210, simply start at 28 = 256, and then double it twice.
You can go the other way as well. If you needed to know what 26 is, you just cut 256 in half two times: once to reach 27 and then one more time to reach 26.
Take, for example, a Class A default subnet mask, which is 255.0.0.0. This means that the first byte of the subnet mask is all ones (1s), or 11111111. When referring to a slash notation, you need to count all the 1s bits to figure out your mask. The 255.0.0.0 is considered a /8 because it has 8 bits that are 1s—that is, 8 bits that are turned on.
A Class B default mask would be 255.255.0.0, which is a /16 because 16 bits are ones (1s): 11111111.11111111.00000000.00000000.
Throughout this chapter you’ll learn several important terms vital to your understanding of the Internet Protocol. Here are a few to get you started:
Bit A bit is one digit, either a 1 or a 0.
Byte A byte is 7 or 8 bits, depending on whether parity is used. For the rest of this chapter, always assume a byte is 8 bits.
Octet An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter, the terms byte and octet are completely interchangeable.
Network address This is the designation used in routing to send packets to a remote network—for example, 10.0.0.0, 172.16.0.0, and 192.168.10.0.
Broadcast address The address used by applications and hosts to send information to all nodes on a network is called the broadcast address. Examples include 255.255.255.255, which is all networks, all nodes; 172.16.255.255, which is all subnets and hosts on network 172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on network 10.0.0.0.
An IP address consists of 32 bits of information. These bits are divided into four sections, referred to as octets or bytes, each containing 1 byte (8 bits). You can depict an IP address using one of three methods:
Dotted-decimal, as in 172.16.30.56
Binary, as in 10101100.00010000.00011110.00111000
Hexadecimal, as in AC.10.1E.38
Address
Function
Network address of all 0s
Interpreted to mean “this network or segment.”
Network address of all 1s
Interpreted to mean “all networks.”
Network 127.0.0.1
Reserved for loopback tests. Designates the local node and allows that node to send a test packet to itself without generating network traffic.
Node address of all 0s
Interpreted to mean “network address” or any host on specified network.
Node address of all 1s
Interpreted to mean “all nodes” on the specified network; for example, 128.2.255.255 means “all nodes” on network 128.2 (Class B address).
Entire IP address set to all 0s
Used by Cisco routers to designate the default route. Could also mean “any network.”
Entire IP address set to all 1s (same as 255.255.255.255)
Broadcast to all nodes on the current network; sometimes called an “all 1s broadcast” or limited broadcast.
In a Class A network address, the first byte is assigned to the network address and the three remaining bytes are used for the node addresses. The Class A format is as follows:
network.node.node.node
For example, in the IP address 49.22.102.70, the 49 is the network address and 22.102.70 is the node address. Every machine on this particular network would have the distinctive network address of 49.
Class A network addresses are 1 byte long, with the first bit of that byte reserved and the 7 remaining bits available for manipulation (addressing). As a result, the maximum number of Class A networks that can be created is 128. Why? Because each of the 7 bit positions can be either a 0 or a 1, thus 27, or 128.
The IP address 127.0.0.1 is used to test the IP stack on an individual node and cannot be used as a valid host address
Here’s an example of how to figure out the valid host IDs in a Class A network address:
All host bits off is the network address: 10.0.0.0.
All host bits on is the broadcast address: 10.255.255.255.
The valid hosts are the numbers in between the network address and the broadcast address: 10.0.0.1 through 10.255.255.254. Notice that 0s and 255s can be valid host IDs. All you need to remember when trying to find valid host addresses is that the host bits can’t all be turned off or all be on at the same time.
In a Class B network address, the first 2 bytes are assigned to the network address and the remaining 2 bytes are used for node addresses. The format is as follows:
network.network.node.node
For example, in the IP address 172.16.30.56, the network address is 172.16 and the node address is 30.56.
With a network address being 2 bytes (8 bits each), there would be 216 unique combinations. But the Internet designers decided that all Class B network addresses should start with the binary digit 1, then 0. This leaves 14 bit positions to manipulate, therefore 16,384 (that is, 214) unique Class B network addresses.
A Class B address uses 2 bytes for node addresses. This is 216 minus the two reserved patterns (all 0s and all 1s), for a total of 65,534 possible node addresses for each Class B network.
Here’s an example of how to find the valid hosts in a Class B network:
All host bits turned off is the network address: 172.16.0.0.
All host bits turned on is the broadcast address: 172.16.255.255.
The valid hosts would be the numbers in between the network address and the broadcast address: 172.16.0.1 through 172.16.255.254.
The first 3 bytes of a Class C network address are dedicated to the network portion of the address, with only 1 measly byte remaining for the node address. Here’s the format:
network.network.network.node
Using the example IP address 192.168.100.102, the network address is 192.168.100 and the node address is 102.
In a Class C network address, the first three bit positions are always the binary 110. The calculation is 3 bytes, or 24 bits, minus 3 reserved positions leaves 21 positions. Hence, there are 221, or 2,097,152, possible Class C networks.
Each unique Class C network has 1 byte to use for node addresses. This leads to 28 or 256, minus the two reserved patterns of all 0s and all 1s, for a total of 254 node addresses for each Class C network.
Here’s an example of how to find a valid host ID in a Class C network:
All host bits turned off is the network ID: 192.168.100.0.
All host bits turned on is the broadcast address: 192.168.100.255.
The valid hosts would be the numbers in between the network address and the broadcast address: 192.168.100.1 through 192.168.100.254.
The people who created the IP addressing scheme also created what we call private IP addresses. These addresses can be used on a private network, but they’re not routable through the Internet. This is designed for the purpose of creating a measure of well-needed security, but it also conveniently saves valuable IP address space.
If every host on every network had to have real routable IP addresses, we would have run out of IP addresses to hand out years ago. But by using private IP addresses, ISPs, corporations, and home users only need a relatively tiny group of bona fide IP addresses to connect their networks to the Internet. This is economical because they can use private IP addresses on their inside networks and get along just fine.
To accomplish this task, the ISP and the corporation—the end user, no matter who they are—need to use something called a Network Address Translation (NAT), which basically takes a private IP address and converts it for use on the Internet. Many people can use the same real IP address to transmit out onto the Internet. Doing things this way saves megatons of address space—good for us all!
Address Class
Reserved address space
Class A
10.0.0.0 through 10.255.255.255
Class B
172.16.0.0 through 172.31.255.255
Class C
192.168.0.0 through 192.168.255.255
The rule of thumb in the consulting world is, when you’re setting up a corporate network— regardless of how small it is—you should use a Class A network address because it gives you the most flexibility and growth options. For example, if you used the 10.0.0.0 network address with a /24 mask, then you’d have 65,536 networks, each with 254 hosts. Lots of room for growth with that network!
But if you’re setting up a home network, you’d opt for a Class C address because it is the easiest for people to understand and configure. Using the default Class C mask gives you one network with 254 hosts—plenty for a home network.
An important thing to remember about DNS is that if you can ping a device with an IP address but cannot use its FQDN, then you might have some type of DNS configuration failure.
But there is a lot of information a DHCP server can provide to a host when the host is requesting an IP address from the DHCP server. Here’s a list of the information a DHCP server can provide:
IP address
Subnet mask
Domain name
Default gateway (routers)
DNS
WINS information
The following sections describe the two protocols at this layer:
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Remember, this is still considered layer 4, and Cisco really likes the way layer 4 can use acknowledgments, sequencing, and flow control.
The TCP header is 20 bytes long, or up to 24 bytes with options. You need to understand what each field in the TCP segment is. The TCP segment contains the following fields:
Source port The port number of the application on the host sending the data. (Port numbers will be explained a little later in this section.)
Destination port The port number of the application requested on the destination host.
Sequence number Puts the data back in the correct order or retransmits missing or damaged data, a process called sequencing.
Acknowledgment number Defines which TCP octet is expected next.
Header length The number of 32-bit words in the TCP header. This indicates where the data begins. The TCP header (even one including options) is an integral number of 32 bits in length.
Reserved Always set to zero.
Code bits Control functions used to set up and terminate a session.
Window The window size the sender is willing to accept, in octets.
Checksum The cyclic redundancy check (CRC), because TCP doesn’t trust the lower layers and checks everything. The CRC checks the header and data fields.
Urgent A valid field only if the Urgent pointer in the code bits is set. If so, this value indicates the offset from the current sequence number, in octets, where the first segment of non-urgent data begins.
Options May be 0 or a multiple of 32 bits, if any. What this means is that no options have to be present (option size of 0). However, if any options are used that do not cause the option field to total a multiple of 32 bits, padding of 0s must be used to make sure the data begins on a 32-bit boundary.
Data Handed down to the TCP protocol at the Transport layer, which includes the upper-layer headers.
The Requests for Comments (RFCs) form a series of notes, started in 1969, about the Internet (originally the ARPAnet). The notes discuss many aspects of computer communication, focusing on networking protocols, procedures, programs, and concepts but also including meeting notes, opinion, and sometimes humor.
There are some situations in which it would definitely be wise for developers to opt for UDP rather than TCP. Remember the watchdog SNMP up there at the Process/Application layer? SNMP monitors the network, sending intermittent messages and a fairly steady flow of status updates and alerts, especially when running on a large network. The cost in overhead to establish, maintain, and close a TCP connection for each one of those little messages would reduce what would be an otherwise healthy, efficient network to a dammed-up bog in no time!
Another circumstance calling for UDP over TCP is when reliability is already handled at the Process/Application layer. Network File System (NFS) handles its own reliability issues, making the use of TCP both impractical and redundant. But ultimately, it’s up to the application developer to decide whether to use UDP or TCP, not the user who wants to transfer data faster.
It’s important for you to understand what each field in the UDP segment is. The UDP segment contains the following fields:
Source port Port number of the application on the host sending the data.
Destination port Port number of the application requested on the destination host.
Length Length of UDP header and UDP data.
Checksum Checksum of both the UDP header and UDP data fields.
Data Upper-layer data.
Sequenced
Unsequenced
Reliable
Unreliable
Connection-oriented
Connectionless
Virtual circuit
Low overhead
Acknowledgments
No acknowledgment
Windowing flow control
No windowing or flow control
The different port numbers that can be used are explained next:
Numbers below 1024 are considered well-known port numbers and are defined in RFC 3232.
Numbers 1024 and above are used by the upper layers to set up sessions with other hosts and by TCP to use as source and destination addresses in the TCP segment.
What makes TCP reliable is sequencing, acknowledgments, and flow control (windowing). UDP does not have reliability.
