Cisco Learning Network Binary Game
Elia Khensamphanh
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The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
If you have already received your legitimate address(es) from the Internet Network Information Center (InterNIC), you are ready to begin. If you do not plan to connect to the Internet, Cisco strongly suggests that you use reserved addresses from RFC 1918.
An IP address is an address used in order to uniquely identify a device on an IP network. The address is made up of 32 binary bits, which can be divisible into a network portion and host portion with the help of a subnet mask. The 32 binary bits are broken into four octets (1 octet = 8 bits). Each octet is converted to decimal and separated by a period (dot). For this reason, an IP address is said to be expressed in dotted decimal format (for example, 172.16.81.100). The value in each octet ranges from 0 to 255 decimal, or 00000000 - 11111111 binary.
Here is how binary octets convert to decimal: The right most bit, or least significant bit, of an octet holds a value of 20. The bit just to the left of that holds a value of 21. This continues until the left-most bit, or most significant bit, which holds a value of 27. So if all binary bits are a one, the decimal equivalent would be 255 as shown here:
These octets are broken down to provide an addressing scheme that can accommodate large and small networks. There are five different classes of networks, A to E. This document focuses on classes A to C, since classes D and E are reserved, and discussion of them is beyond the scope of this document.
Given an IP address, its class can be determined from the three high-order bits (the three left-most bits in the first octet). Figure 1 shows the significance in the three high order bits and the range of addresses that fall into each class. For informational purposes, Class D and Class E addresses are also shown.
In a Class B address, the first two octets are the network portion, so the Class B example in Figure 1 has a major network address of 128.0.0.x - 191.255.255.x. Octets 3 and 4 (16 bits) are for local subnets and hosts. Class B addresses are used for networks that have between 256 and 65534 hosts.
In a Class C address, the first three octets are the network portion. The Class C example in Figure 1 has a major network address of 192.0.0.x - 223.255.255.x. Octet 4 (8 bits) is for local subnets and hosts - perfect for networks with less than 254 hosts.
A network mask helps you know which portion of the address identifies the network and which portion of the address identifies the node. Class A, B, and C networks have default masks, also known as natural masks, as shown here:
An IP address on a Class A network that has not been subnetted would have an address/mask pair similar to: 10.20.15.1 255.0.0.0. In order to see how the mask helps you identify the network and node parts of the address, convert the address and mask to binary numbers.
Once you have the address and the mask represented in binary, then identification of the network and host ID is easier. Any address bits which have corresponding mask bits set to 1 represent the network ID. Any address bits that have corresponding mask bits set to 0 represent the node ID.
Subnetting allows you to create multiple logical networks that exist within a single Class A, B, or C network. If you do not subnet, you are only able to use one network from your Class A, B, or C network, which is unrealistic.
Each data link on a network must have a unique network ID, and every node on that link is a member of the same network. If you break a major network (Class A, B, or C) into smaller subnetworks, it allows you to create a network of interconnecting subnetworks. Each data link on this network would then have a unique network/subnetwork ID. Any device, or gateway, that connects n networks/subnetworks has n distinct IP addresses, one for each network / subnetwork that it interconnects.
In order to subnet a network, extend the natural mask with some of the bits from the host ID portion of the address in order to create a subnetwork ID. For example, given a Class C network of 192.168.5.0 which has a natural mask of 255.255.255.0, you can create subnets in this manner:
By extending the mask to be 255.255.255.224, you have taken three bits (indicated by sub) from the original host portion of the address and used them to make subnets. With these three bits, it is possible to create eight subnets. With the other five host ID bits, each subnet can have up to 32 host addresses, 30 of which can actually be assigned to a device since host ids of all zeros or all ones are not allowed (it is very important to remember this). So, with this in mind, these subnets have been created.
Notice that each of the routers in Figure 2 is attached to four subnetworks, one subnetwork is common to both routers. Also, each router has an IP address for each subnetwork to which it is attached. Each subnetwork could potentially support up to 30 host addresses.
Since you now have four bits to make subnets with, you only have four bits left for host addresses. So in this case you can have up to 16 subnets, each of which can have up to 16 host addresses (14 of which can be assigned to devices).
Look at how a Class B network can be subnetted. If you have network 172.16.0.0, then you know that its natural mask is 255.255.0.0 or 172.16.0.0/16. Extending the mask to anything beyond 255.255.0.0 means you are subnetting. You can quickly see that you have the ability to create a lot more subnets than with the Class C network. If you use a mask of 255.255.248.0 (/21), how many subnets and hosts per subnet does this allow for?
You use five bits from the original host bits for subnets. This allows you to have 32 subnets (25). When the five bits for subnetting are used, you are left with 11 bits for host addresses. This allows each subnet so have 2048 host addresses (211), 2046 of which could be assigned to devices.
Now that you understand subnetting, put this knowledge to use. In this example, you are given two address / mask combinations, written with the prefix/length notation, which have been assigned to two devices. Your task is to determine if these devices are on the same subnet or different subnets. You can use the address and mask of each device in order to determine to which subnet each address belongs.
Look at the address bits that have a corresponding mask bit set to one, and set all the other address bits to zero (this is equivalent to when you perform a logical AND between the mask and address). It shows you to which subnet this address belongs. In this case, Device A belongs to subnet 172.16.16.0.
As you look at the network shown in Figure 3, you can see that you are required to create five subnets. The largest subnet must support 28 host addresses. Is this possible with a Class C network? And if so, then how?
In order to start, look at the subnet requirement. In order to create the five needed subnets, you would need to use three bits from the Class C host bits. Two bits would only allow you four subnets (22).
In all of the previous examples of subnetting, notice that the same subnet mask was applied for all the subnets. This means that each subnet has the same number of available host addresses. You need this in some cases, but, in most cases when there is the same subnet mask for all subnets, it wastes address space. For example, in the Sample Exercise 2 section, a class C network was split into eight equal-size subnets; however, each subnet did not utilize all available host addresses, which resulted in wasted address space. Figure 4 illustrates this wasted address space.
Figure 4 illustrates which of the subnets used. NetA, NetC, and NetD have a lot of unused host address space. It is possible that this was a deliberate design accounting for future growth, but in many cases, this is just wasted address space due to the fact that the same subnet mask is used for all the subnets.
Classless Interdomain Routing (CIDR) was introduced in order to improve both address space utilization and routing scalability in the Internet. It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers.
A 30-bit subnet mask allows for four IPv4 addresses: two host addresses, one all-zeros network, and one all-ones broadcast address. A point-to-point link can only have two host addresses. There is no real need to have the broadcast and all-zeros addresses with point-to-point links. A 31-bit subnet mask allows for exactly two host addresses, and eliminates the broadcast and all-zeros addresses, thus it conserves the use of IP addresses to the minimum for point-to-point links.
The /31 subnet can be used on true point-to-point links, such as serial or POS interfaces. However, they can also be used on broadcast interface types like ethernet interfaces. If that is the case, make sure there are only two IPv4 addresses needed on that ethernet segment.
A subnet mask of 255.255.255.255 (a /32 subnet) describes a subnet with only one IPv4 host address. These subnets cannot be used to assign address to network links because they always need more than one address per link. The use of /32 is strictly reserved for use on links that can have only one address. The example for Cisco routers is the loopback interface. These interfaces are internal interfaces and do not connect to other devices. As such, they can have a /32 subnet.
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