Obtaining an IP address
Friday, October 2, 2009
| A network host needs to obtain a globally unique address in order to function on the Internet. The physical or MAC address that a host has is only locally significant, identifying the host within the local area network. Since this is a Layer 2 address, the router does not use it to forward outside the LAN. IP addresses are the most commonly used addresses for Internet communications. This protocol is a hierarchical addressing scheme that allows individual addresses to be associated together and treated as groups. These groups of addresses allow efficient transfer of data across the Internet. Network administrators use two methods to assign IP addresses. These methods are static and dynamic. Later in this lesson, static addressing and three variations of dynamic addressing will be covered. Regardless of which addressing scheme is chosen, no two interfaces can have the same IP address. Two hosts that have the same IP address could create a conflict that might cause both of the hosts involved not to operate properly. As shown in Figure |
Static assignment works best on small, infrequently changing networks. The system administrator manually assigns and tracks IP addresses for each computer, printer, or server on the intranet. Good recordkeeping is critical to prevent problems which occur with duplicate IP addresses. This is possible only when there are a small number of devices to track.
Servers should be assigned a static IP address so workstations and other devices will always know how to access needed services. Consider how difficult it would be to phone a business that changed its phone number every day.
Other devices that should be assigned static IP addresses are network printers, application servers, and routers.
RARP IP 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. |
BOOTP IP address assignment
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 |
| 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. |
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.
Address Resolution Protocol (ARP)
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.
Cisco Systems, Inc.
Internet Addresses
| For any two systems to communicate, they must be able to identify and locate each other. While these addresses in Figure are not actual network addresses, they represent and show the concept of address grouping. This uses the A or B to identify the network and the number sequence to identify the individual host. A computer may be connected to more than one network. In this situation, the system must be given more than one address. Each address will identify the connection of the computer to a different network. A device is not said to have an address, but that each of the connection points, or interfaces, on that device has an address to a network. This will allow other computers to locate the device on that particular network. The combination of letter (network address) and the number (host address) create a unique address for each device on the network. Each computer in a TCP/IP network must be given a unique identifier, or IP address. This address, operating at Layer 3, allows one computer to locate another computer on a network. All computers also have a unique physical address, known as a MAC address. These are assigned by the manufacturer of the network interface card. MAC addresses operate at Layer 2 of the OSI model. An IP address is a 32-bit sequence of 1s and 0s. Figure Using dotted decimal allows number patterns to be more easily understood. Both the binary and decimal numbers in Figure It is easy to see the relationship between the numbers 192.168.1.8 and 192.168.1.9, where 11000000.10101000.00000001.00001000 and 11000000.10101000.00000001.00001001 are not as easy to recognize. Looking at the binary, it is almost impossible to see that they are consecutive numbers |
There are many ways to solve a problem. There are also several ways to convert decimal numbers to binary numbers. One method is presented here, however it is not the only method. The student may find other methods easier. It is a matter of personal preference.
When converting a decimal number to binary, the biggest power of two that will fit into the decimal number must be determined. If this process is designed to be working with computers, the most logical place to start is with the largest values that will fit into a byte or two bytes. As mentioned earlier, the most common grouping of bits is eight, which make up one byte. However, sometimes the largest value that can be held in one byte is not large enough for the values needed. To accommodate this, bytes are combined. Instead of having two eight-bit numbers, one 16-bit number is created. Instead of three eight-bit numbers, one 24-bit number is created. The same rules apply as they did for eight-bit numbers. Multiply the previous position value by two to get the present column value.
Since working with computers often is referenced by bytes it is easiest to start with byte boundaries and calculate from there. Start by calculating a couple of examples, the first being 6,783. Since this number is greater than 255, the largest value possible in a single byte, two bytes will be used. Start calculating from 215. The binary equivalent of 6,783 is 00011010 01111111.
The second example is 104. Since this number is less than 255, it can be represented by one byte. The binary equivalent of 104 is 01101000.
This method works for any decimal number. Consider the decimal number one million. Since one million is greater than the largest value that can be held in two bytes, 65535, at least three bytes will be needed. By multiplying by two until 24 bits, three bytes, is reached, the value will be 8,388,608. This means that the largest value that 24 bits can hold is 16,777,215. So starting at the 24-bit, follow the process until zero is reached. Continuing with the procedure described, it is determined that the decimal number one million is equal to the binary number 00001111 01000010 01000000.
Figure includes some decimal to binary conversion exercises.
Binary to decimal conversion is just the opposite. Simply place the binary in the table and if there is a one in a column position add that value into the total. Convert 00000100 00011101 to decimal. The answer is 1053.
Figure includes some decimal to binary conversion exercises.
IPv4 addressing
A router forwards packets from the originating network to the destination network using the IP protocol. The packets must include an identifier for both the source and destination networks. Using the IP address of destination network, a router can deliver a packet to the correct network. When the packet arrives at a router connected to the destination network, the router uses the IP address to locate the particular computer connected to that network. This system works in much the same way as the national postal system. When the mail is routed, it must first be delivered to the post office at the destination city using the zip code. That post office then must locate the final destination in that city using the street address. This is a two-step process.
Accordingly, every IP address has two parts. One part identifies the network where the system is connected, and a second part identifies that particular system on the network. As is shown Figure , each octet ranges from 0 to 255. Each one of the octets breaks down into 256 subgroups and they break down into another 256 subgroups with 256 addresses in each. By referring to the group address directly above a group in the hierarchy, all of the groups that branch from that address can be referenced as a single unit.
This kind of address is called a hierarchical address, because it contains different levels. An IP address combines these two identifiers into one number. This number must be a unique number, because duplicate addresses would make routing impossible. The first part identifies the system's network address. The second part, called the host part, identifies which particular machine it is on the network.
IP addresses are divided into classes to define the large, medium, and small networks. Class A addresses are assigned to larger networks. Class B addresses are used for medium-sized networks, and Class C for small networks. The first step in determining which part of the address identifies the network and which part identifies the host is identifying the class of an IP addressClass A, B, C, D, and E IP addresses
To accommodate different size networks and aid in classifying these networks, IP addresses are divided into groups called classes.
This is known as classful addressing. Each complete 32-bit IP address is broken down into a network part and a host part. A bit or bit sequence at the start of each address determines the class of the address. There are five IP address classes as shown in Figure 
. The Class A address was designed to support extremely large networks, with more than 16 million host addresses available. Class A IP addresses use only the first octet to indicate the network address. The remaining three octets provide for host addresses.
The first bit of a Class A address is always 0. With that first bit a 0, the lowest number that can be represented is 00000000, decimal 0. The highest number that can be represented is 01111111, decimal 127. The numbers 0 and 127 are reserved and cannot be used as network addresses. Any address that starts with a value between 1 and 126 in the first octet is a Class A address.
The 127.0.0.0 network is reserved for loopback testing. Routers or local machines can use this address to send packets back to themselves. Therefore, this number cannot be assigned to a network.
The Class B address was designed to support the needs of moderate to large-sized networks. A Class B IP address uses the first two of the four octets to indicate the network address. The other two octets specify host addresses.
The first two bits of the first octet of a Class B address are always 10. The remaining six bits may be populated with either 1s or 0s. Therefore, the lowest number that can be represented with a Class B address is 10000000, decimal 128. The highest number that can be represented is 10111111, decimal 191. Any address that starts with a value in the range of 128 to 191 in the first octet is a Class B address.
The Class C address space is the most commonly used of the original address classes. This address space was intended to support small networks with a maximum of 254 hosts.
A Class C address begins with binary 110. Therefore, the lowest number that can be represented is 11000000, decimal 192. The highest number that can be represented is 11011111, decimal 223. If an address contains a number in the range of 192 to 223 in the first octet, it is a Class C address.
The Class D address class was created to enable multicasting in an IP address. A multicast address is a unique network address that directs packets with that destination address to predefined groups of IP addresses. Therefore, a single station can simultaneously transmit a single stream of data to multiple recipients.
The Class D address space, much like the other address spaces, is mathematically constrained. The first four bits of a Class D address must be 1110. Therefore, the first octet range for Class D addresses is 11100000 to 11101111, or 224 to 239. An IP address that starts with a value in the range of 224 to 239 in the first octet is a Class D address.
A Class E address has been defined. 
However, the Internet Engineering Task Force (IETF) reserves these addresses for its own research. Therefore, no Class E addresses have been released for use in the Internet. The first four bits of a Class E address are always set to 1s. Therefore, the first octet range for Class E addresses is 11110000 to 11111111, or 240 to 255.
shows the IP address range of the first octet both in decimal and binary for each IP address classReserved IP addresses
Certain host addresses are reserved and cannot be assigned to devices on a network. These reserved host addresses include the following:
- Network address – Used to identify the network itself
In Figure
- Broadcast address – Used for broadcasting packets to all the devices on a network
In Figure
An IP address that has binary 0s in all host bit positions is reserved for the network address. In a Class A network example, 113.0.0.0 is the IP address of the network, known as the network ID, containing the host 113.1.2.3. A router uses the network IP address when it forwards data on the Internet. In a Class B network example, the address 176.10.0.0 is a network address, as shown in Figure .
In a Class B network address, the first two octets are designated as the network portion. The last two octets contain 0s because those 16 bits are for host numbers and are used to identify devices that are attached to the network. The IP address, 176.10.0.0, is an example of a network address. This address is never assigned as a host address. A host address for a device on the 176.10.0.0 network might be 176.10.16.1. In this example, “176.10” is the network portion and “16.1” is the host portion.
To send data to all the devices on a network, a broadcast address is needed. A broadcast occurs when a source sends data to all devices on a network. To ensure that all the other devices on the network process the broadcast, the sender must use a destination IP address that they can recognize and process. Broadcast IP addresses end with binary 1s in the entire host part of the address.
The broadcast that would be sent out to all devices on that network would include a destination address of 176.10.255.255. This is because 255 is the decimal value of an octet containing 11111111Public and private IP addresses
The stability of the Internet depends directly on the uniqueness of publicly used network addresses. In Figure
, there is an issue with the network addressing scheme. In looking at the networks, both have a network address of 198.150.11.0. The router in this illustration will not be able to forward the data packets correctly. Duplicate network IP addresses prevent the router from performing its job of best path selection. Unique addresses are required for each device on a network. A procedure was needed to make sure that addresses were in fact unique. Originally, an organization known as the Internet Network Information Center (InterNIC) handled this procedure. InterNIC no longer exists and has been succeeded by the Internet Assigned Numbers Authority (IANA). IANA carefully manages the remaining supply of IP addresses to ensure that duplication of publicly used addresses does not occur. Duplication would cause instability in the Internet and compromise its ability to deliver datagrams to networks.
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. All machines connected to the Internet agree to conform to the system. Public IP addresses must be obtained from an Internet service provider (ISP) or a registry at some expense.
With the rapid growth of the Internet, public IP addresses were beginning to run out. New addressing schemes, such as classless interdomain routing (CIDR) and IPv6 were developed to help solve the problem. CIDR and IPv6 are discussed later in the course.
Private IP addresses are another solution to the problem of the impending exhaustion of public IP addresses. As mentioned, public networks require hosts to have unique IP addresses. However, 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. Many private networks exist alongside public networks. However, a private network using just any address is strongly discouraged because that network might eventually be connected to the Internet. RFC 1918 sets aside three blocks of IP addresses 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 not routed on the Internet backbone. Internet routers immediately discard private addresses. If addressing a nonpublic intranet, a test lab, or a home network, these private addresses can be used instead of globally unique addresses. Private IP addresses can be intermixed, as shown in the graphic, with public IP addresses. This will conserve the number of addresses used for internal connections.
Connecting a network using private addresses to the Internet requires translation of the private addresses to public addresses. This translation process is referred to as Network Address Translation (NAT). A router usually is the device that performs NAT. NAT, along with CIDR and IPv6 are covered in more depth later in the curriculum.
Introduction to subnetting
Subnetting is another method of managing IP addresses. This method of dividing full network address classes into smaller pieces has prevented complete IP address exhaustion. It is impossible to cover TCP/IP without mentioning subnetting. As a system administrator it is important to understand subnetting as a means of dividing and identifying separate networks throughout the LAN. It is not always necessary to subnet a small network. However, for large or extremely large networks, subnetting is required. 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. An example would be the U.S. telephone system which is broken into area codes, exchange codes, and local numbers.
The system administrator must resolve these issues when adding and expanding the network. It is important to know how many subnets or networks are needed and how many hosts will be needed on each network. With subnetting, the network is not limited to the default Class A, B, or C network masks and there is more flexibility in the network design.
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. The ability to decide how to divide the original host portion into the new subnet and host fields provides addressing flexibility for the network administrator.
To create a subnet address, a network administrator borrows bits from the host field and designates them as the subnet field. The minimum number of bits that can be borrowed is two. When creating a subnet, where only one bit was borrowed the network number would be the .0 network. The broadcast number would then be the .255 network. The maximum number of bits that can be borrowed can be any number that leaves at least two bits remaining, for the host numberIPv4 versus IPv6
| When TCP/IP was adopted in the 1980s, it relied on a two-level addressing scheme. At the time this offered adequate scalability. Unfortunately, the designers of TCP/IP could not have predicted that their protocol would eventually sustain a global network of information, commerce, and entertainment. Over twenty years ago, IP Version 4 (IPv4) offered an addressing strategy that, although scalable for a time, resulted in an inefficient allocation of addresses. The Class A and B addresses make up 75 percent of the IPv4 address space, however fewer than 17,000 organizations can be assigned a Class A or B network number. Unfortunately, Class C addresses are limited to 254 usable hosts. This does not meet the needs of larger organizations that cannot acquire a Class A or B address. Even if there were more Class A, B, and C addresses, too many network addresses would cause Internet routers to come to a stop under the burden of the enormous size of routing tables required to store the routes to reach each of the networks. As early as 1992, the Internet Engineering Task Force (IETF) identified the following two specific concerns:
Over the past two decades, numerous extensions to IPv4 have been developed. These extensions are specifically designed to improve the efficiency with which the 32-bit address space can be used. Two of the more important of these are subnet masks and classless interdomain routing (CIDR), which are discussed in more detail in later lessons. Meanwhile, an even more extendible and scalable version of IP, IP Version 6 (IPv6), has been defined and developed. After years of planning and development, IPv6 is slowly being implemented in select networks. Eventually, IPv6 may replace IPv4 as the dominant Internet protocol. |
Cisco Systems, Inc.
