Wednesday, April 25, 2018

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Internet Protocol (IP)

I remember first learning about the world-wide Internet, and wondering what it actually looked like. The first vision entering my mind when people told me about a computer network spanning nearly all of the United States and many other parts of the world was that of a thick cable strung along telephone poles and buried underground, with a big sign on it saying “Internet.” I also remember well the shock of learning that although the Internet made use of several high-capacity networks (called backbones) connecting large data centers in different cities, the real “magic” of the Internet did not reside in any particular cable or link. Instead, what made the Internet so widespread and accessible was actually a protocol allowing for the free exchange of data along and between disparate systems. This “protocol” allowed digital data to be packaged in such a way that it could be sent long nearly any kind of communications link (from copper wires to fiber-optic to radio waves) – and indeed along multiple pathways between the same two points – while arriving at the destination intact. Thus, the Internet was akin to a random patchwork of existing communications pathways pressed into coordinated service by the sharing of a common “language.” In this section, we will investigate the protocol at the heart of the Internet, appropriately called Internet Protocol, or IP.

Physical network standards such as Ethernet only define aspects relevant to lower layers of the OSI Reference Model. While these details are essential for communication to occur, they are not enough on their own to support a wide-spread communications system. For this reason, network standards such as EIA/TIA-485 and Ethernet almost always comprise the lower layer(s) of a more complex communications protocol capable of managing higher-order addresses, message integrity, “sessions” between computers, and a host of other details.

Internet Protocol (IP) manages network addresses and data handling over a much larger physical domain than Ethernet is able to. The basic principle of IP is that large messages are broken down into packets transmitted individually and received individually (then reassembled at the receiver to form the original, complete message). An analogy for this process might be an author with a printed paper manuscript for a book, who needs to get her manuscript to a print shop across town. Unfortunately, the mail service in this town cannot handle the bulky manuscript in one piece, so the author divides the manuscript into bundles of 10 pages each and mails each of these bundles to the print shop, with instructions in each envelope on how to re-assemble the bundles into the complete book. The individual bundles may not make it to the print shop on the same day, or even in the correct order, but the instructions contained within each one make it possible for the people at the print shop to reassemble the entire manuscript once all the bundles have arrived.

This strategy for transmitting large digital messages is at the heart of the Internet: data sent from one computer to another over the internet is first broken down into packets, which are then sent and routed over a variety of pathways to their destination. The receiving computer then reassembles the packets into the original form. This “fragmentation” of data may seem unnecessary, but it actually provides a great deal of flexibility in how data is routed from one point to another.


IP addresses

IP is a “layer 3” technology, being concerned with network-wide addresses for routing information between two different locations. IP is not concerned with the details of communication along any particular wire or fiber-optic cable. It is not “aware” of how bits are represented electrically, or what kind of connectors are used to couple cables together. IP is only concerned with “networks” in the broad sense of the word, as abstract collections of computers that somehow (it doesn’t care exactly how) are connected to each other.

Networking equipment (DCE) designed to pay attention to IP addresses for routing purposes are called, not surprisingly, routers. Their purpose is to direct packets to their appropriate destinations in the shortest amount of time. In order for the Internet Protocol to specify where packets are coming from and where they are going to, each source and destination must bear its own IP address. IP version 4 (IPv4) uses 32-bit addresses, usually expressed as four octets written using decimal numbers. For example:

     IP address 00000000 00000000 00000000 00000000 is written as

     IP address 11111111 11111111 11111111 11111111 is written as

     IP address 10101001 11111010 00101101 00000011 is written as

In order for two inter-connected computers to exchange data using Internet Protocol, each one must have a unique IP address:


Computer to Computer Ethernet Connectivity Using IP Addressing


At first, this may seem redundant. Doesn’t each and every Ethernet device already have its own unique “MAC address” 48 bits in length to distinguish it from every other Ethernet device in existence? If so, why add another set of identifying addresses to the system?

This is true – Ethernet devices are already uniquely addressed – but those MAC addresses serve different purposes than IP addresses. Recall that Ethernet is a standard only at layers 1 and 2, and is not “aware” of any higher-level concerns. Ethernet MAC addresses are useful to switching hubs and other Ethernet DCE devices tasked with management of Ethernet data frames, but those MAC addresses – unique as they may be – have little relevance in the greater picture of IP where we must fragment and reassemble messages over very large-scale networks. More importantly, the reason we need IP addresses is to be able to use interconnecting networks other than Ethernet. For example, two computers may be connected to each other with a simple EIA/TIA-232 cable (or even using radio transceiver units for a “wireless” connection) instead of Ethernet, but still use Internet Protocol to break up large messages and reassemble them at the receiving end1. By having its own dedicated addressing scheme, IP ensures computers may be able to disassemble data into packets, send those packets, receive those packets, then re-assemble the packets into the original data regardless of the physical interconnection details, channel arbitration methods, or anything else in between. In a sense, IP is the “glue” that holds disparate networks together, and makes something like a world-wide Internet possible when so many different network types exist to connect digital devices together.

A helpful analogy is to think of Ethernet MAC addresses like Social Security numbers for United States citizens. Each US citizen should have their own Social Security number, unique to all living persons. This number is used for many purposes, including identification on Federal tax documents, to help route specific information (such as income records and Social Security payments) to the proper people. Despite the uniqueness of these numbers, though, people still need separate mailing addresses in order to receive mail through the postal service and other package distribution agencies. The mailing address serves a different purpose than the Social Security “address” each US citizen possesses. Furthermore, the existence of separate mailing addresses ensures even non-citizens living in the United States (e.g. foreign students, ambassadors, etc.) who have no Social Security numbers still have a way to send and receive mail.

Given the addressing purpose of Internet Protocol (to designate addresses over an extremely large collection of digital communication devices), addresses must be chosen with care. IP version 4 uses a 32-bit field to designate addresses, limiting its address capacity to 232 unique addresses. As large as this number is, it is not enough to uniquely identify all Internet-capable devices worldwide. The inventors of IP did not dream their Internet would grow to the proportions it has today. Let this be a lesson to all those involved with computers: the future will always be bigger than you think! A variety of clever techniques has been developed to deal with this shortage of IP addresses.

One of them is to dynamically assign addresses to Internet-connected computers only when they are turned on. This is how most personal Internet connections work: when you boot up your personal computer to connect to the Internet, your service provider assigns you a temporary IP address through a protocol called DHCP (Dynamic Host Configuration Protocol). Your provider then forces you to relinquish this temporary IP address when you shut down your computer, so someone else may use it for theirs.

The Internet Corporation for Assigned Names and Numbers, or ICANN, is the organization responsible2 for assigning IP addresses to Internet users worldwide (among other tasks). This group has designated certain IP address ranges specific to internal (i.e. Local Area Network, or LAN) network devices, which shall never be used “publicly” to address devices on the world-wide Internet. These specially-designated “private” LAN address ranges are as follows: to to to

Additionally, all computers have their own special loopback IP address, used to send IP message packets to itself for certain purposes (including diagnostics): This IP address is completely virtual, not associated with any network hardware at all3. Therefore, the ping command executed on any computer should always be able to detect address, regardless of the status or even existence of actual network hardware (cards or interfaces) on that computer. Failure of the ping command to detect the loopback address is a sign that the computer’s operating system is not configured to use Internet Protocol.

A computer’s loopback address may have uses other than diagnostic. Some computer applications are network-oriented by nature, and rely on IP addresses even if the application is performing some local function rather than a function between computers on an actual network. The X-windows graphic-user interface (GUI) system popularly used on UNIX operating systems is an example of this, referencing the loopback address to form a connection between client and server applications running on the same computer.


Subnetworks and subnet masks

IP (version 4) addresses are used in conjunction with something called subnet masks4 to divide IP networks into “subnetworks.” A “subnetwork” is a range of IP-addressed devices allowed to communicate with each other. You may think of the subnet mask to be a sort of “filter” used to identify IP addresses not belonging to the proper range.

The subnet mask works as a bitwise filter, identifying those bits in the IP address defining the subnetwork. For example, if the subnet mask on a computer is set to (binary 11111111 00000000 00000000 00000000), it means the first 8 bits of the IP address define the subnetwork, and thus the computer is only allowed to communicate with another computer belonging to the same subnetwork (i.e. having the same first octet in its IP address).

A set of examples showing two interconnected computers with differing IP addresses (and in some cases, different masks) illustrates how this works5. In the first example, two computers with IP addresses differing in the last two octets are able to communicate because their subnets are the same (169.254):


Two computers with IP addresses differing in the last two octets are able to communicate because their subnets are the same.

We may check to see the IP addresses and subnet masks are correct by using a command-line program called ping, available on nearly all computer systems. A screenshot of ping being used on a personal computer running the Microsoft Windows XP operating system is shown here:


A Screenshot of ping being used on a personal computer running the Microsoft Windows XP operating system.

The ping utility works by sending a very short digital message to the specified IP address, requesting a reply from that computer. There are usually multiple attempts, with four being shown in this particular example. In fact, it is common among networking professionals to use the word “ping” as a verb, as in “I tried to ping that computer, but it gave no response.”

In the next example, we see two computers with the same mask value, but with different address values in the octets designated by their masks. In other words, these two computers belong to different subnets: one to 167.254 and the other to 169.254, and as a result they are not allowed to communicate with each other using Internet Protocol. The resulting error messages generated by the ping utility are shown in this diagram:


Two computers on different subnets are not allowed to communicate with each other using internet protocol.


In the last example, we see two computers having different mask values as well as different IP addresses. The subnet of the left-hand computer is 169.254.10 while the subnet of the right-hand computer is 169.254:


Two Computers Having Different Mask Values as Well as Different IP Address


The computer on the left may only communicate with IP addresses matching in the first three octets (169.254.10). Seeing that the destination address for the second computer does not match in its third octet, ping returns a “Destination host unreachable” error message when executed from the left-hand computer.

When the computer on the right attempts to communicate with (“ping”) the computer on the left, it is allowed to transmit to that computer because its mask only screens for agreement in the first two octets (169.254), which happen to match. However, the computer on the left is not allowed to transmit to the computer on the right because of its more restrictive subnet, and so ping running on the right-hand computer returns a “Request timed out” error message because it never receives a reply from the left-hand computer to any of its queries.

With just two computers connected by a single cable, the concept of subnetworks and masks seems useless, and indeed it is on this small of a scale. However, “subnetting” is a useful technique for managing high traffic loads on large networked systems using IP addresses, and so it is commonly seen in many local area networks (LANs) such as those found at industry and commercial sites. Another use of ping is to search for unknown IP addresses on a known subnet. This may be done by “pinging” to the broadcast address for that subnet: an IP address formed by the known subnet numbers, followed by all binary 1’s filling the unknown bit spaces. For example, you could use ping to search for devices on the subnet 156.71 (subnet mask by using the following command:



IP version 6

The next version of IP (version 6, or IPv6) uses 128-bit addresses, giving 2128 address possibilities (in excess of 3.4 × 1038), in stark contrast to IPv4’s paltry 232 address space. To put this enormous quantity into perspective, there are enough IPv6 addresses to designate nearly 57 billion of them for each and every gram of the Earth’s mass6. While IPv4 addresses are typically written as four octets in decimal form (e.g., this notation would be very cumbersome for writing IPv6 addresses. Thus, IPv6 addresses are written as a set of eight hexadecimal numbers (up to four characters per number) separated by colons, such as 4ffd:522:c441:d2:93b2:f5a:8:101f. The phase-in of IPv6 to replace IPv4 has already started for certain portions of the Internet, but the full transition to IPv6 is expected to take many years. The IPv6 “loopback” virtual address for computers is 0:0:0:0:0:0:0:1, or more simply written as ::1.



The acronym DNS actually stands for two related things: Domain Name System and Domain Name Server. The first meaning of “DNS” refers to the system of exchanging numerical IP addresses with alphanumeric Uniform Resource Locators (URLs) which are easier for human beings to remember. When you use web browser software to navigate to a web site on the Internet, you have the option of entering the URL name of that site (e.g. or a numerical IP address (e.g. Special computers connected to the Internet called Domain Name Servers, and Domain Name Resolvers (DNRs) use the Address Resolution Protocol (ARP) to convert your target web site name to its actual IP address so that a connection may be made between that computer and yours.

ICANN, the same organization responsible for allotting IP addresses, also maintains databases for all registered domain names.


Command-line diagnostic utilities

In addition to ping, another utility program useful for troubleshooting network connections from a computer’s command line interface is ipconfig. When executed, ipconfig returns a listing of all available (configured and operating) network interfaces on that computer:


Windows IPCONFIG command screenshot


The equivalent command for UNIX operating systems is ifconfig, shown in this screenshot:


Unix IFCONFIG Command screenshot


Some of the more interesting details contained in the output from ifconfig are the IPv6 addresses (in addition to IPv4 addresses), and details on the “loopback” address (IPv4 or IPv6 ::1).

A utility intended to reveal the DNS name of a computer given its IP address, or visa versa, is nslookup. The same command works on Microsoft Windows and UNIX operating systems alike. Here, we see the UNIX version used to identify four IP addresses of the popular Google search engine web site:


Unix version of nslookup screenshot

Another utility used to explore network connections is traceroute (spelled tracert on Microsoft Windows operating systems). This utility sends a test packet to the designated destination address, returning information on all the “hops” the IP packet takes between computers along the network to reach its destination and the amount of time taken to make the trip. Execution of traceroute on a UNIX computer and tracert on a Microsoft Windows computer are shown here:


Unix version of Traceroute screenshot



1In fact, this is precisely the state of affairs if you use a dial-up telephone connection to link your personal computer with the Internet. If you use dial-up, your PC may not use Ethernet at all to make the connection to your telephone provider’s network, but rather it might uses EIA/TIA-232 or USB to a modem (modulator/demodulator) device, which turns those bits into modulated waveforms transmittable over a voice-quality analog telephone line.

2Prior to ICANN’s formation in 1999, the Internet Assigned Numbers Authority, or IANA was responsible for these functions. This effort was headed by a man named Jon Postel, who died in 1998.

3The term “loopback” refers to an old trick used by network technicians to diagnose suspect serial port connections on a computer. Using a short piece of copper wire (or even a paperclip) to “jumper” pins 2 and 3 on an EIA/TIA-232 serial port, any serial data transmitted (out of pin 3) would be immediately received (in pin 2), allowing the serial data to “loop back” to the computer where it could be read. This simple test, if passed, would prove the computer’s low-level communication software and hardware was working properly and that any networking problems must lie elsewhere.

4Also called “netmasks” or simply “masks.”

5These are real test cases I performed between two computers connected on a 10 Mbps Ethernet network. The error messages are those generated by the ping utility when communication was attempted between mis-matched computers.

6According to Douglas Giancoli’s Physics for Scientists and Engineers textbook, the mass of the Earth is 5.98×1024 kg, or 5.98 × 1027 grams. Dividing 2128 (the number of unique IPv6 addresses) by the Earth’s mass in grams yields the number of available IPv6 address per gram of Earth mass. Furthermore, if we assume a grain of sand has a mass of about 1 milligram, and that the Earth is modeled as a very large collection of sand grains (not quite the truth, but good enough for a dramatic illustration!), we arrive at 57 million IPv6 addresses per grain of sand on Earth.


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