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NOTE:  Items highlighted in RED are defined elsewhere in this Glossary, while items highlighted in BLUE are site links for further information.

Public” vs. “Private”, “External” vs. “Internal” IP Addresses…What’s This All About?

This question, which has generated lots of confusion among computer users, is coming up much more frequently as always-on (CABLE, ISDN, DSL, FIOS) networks proliferate and remote desktop connections (“RDPs” such as LogMeIn, TeamViewer, VNC or PCAnywhere) become more prevalent.  ISP service support techs will usually ask you or try to ascertain whether the problem you are experiencing is on the “public” or the “private” side of the network.  Here’s why:  The reason they do that is because the provider’s “service” and therefore it’s support for that service ends at either the DEMARC (where the connection enters your home or office) or at the end of the “public” side of the network (the cable/DSL/FIOS modem), depending on the provider.  They won’t have anything to do with the “private” side of the network. 

Your computer has an IP address that allows you to connect to your internal network.  Now, you say, you’re just a home user and don’t want or have a network like an office. Wrong - you do.  You are “networked” with the cable company or the phone company (if you use DSL).  You do have an IP, although it changes every so often (as we say, it’s “lease” expires). This is done through the CABLE/DSL/FIOS modem furnished by your ISP or your router or both.  Your computer’s internal IP address will look something like this: or 10.1.1.x .  All IP addresses look  the same: four sets of up to three digits, separated by periods.  This IP numbering system is referred to as “IPv4” and is comprised of 4,294,967,296 possible addresses.  Because this limit currently reaching its limit, a new system, IPv6, which has 340,282,366,920,938,463,374,607,431,768,211,456 possible addresses is already in use.  [Click HERE for information about the IPv4 - IPv6 transition.] 

Here’s how it works:  IP specifies the formation of packets (also known as datagrams) to be sent over the Internet as well as an addressing scheme.  IP is usually combined with another higher-level protocol such as TCP (“Transfer Control Protocol”) which establishes a connection between the sender and the receiver.  It’s like the postal system:  IP lets you address mail and drop it off in the mail system, then TCP delivers it to the right address.

An appropriate comparison to the IP address system would be the telephone system.  [Or the mail system, take your pick.]  In the phone system, you are assigned an individual telephone number that is exclusively yours.  To find that number, someone who wants to call you must either have that number (perhaps you gave it to them) or call information (“411”).  Directory assistance will then sort through all issued telephone numbers and people (eliminating your number, but with different area codes; also people with your name but living at a different address) until only you will be left to be found.

The Internet is similar.  You start with either a numeric IP address (as discussed above) or a “Fully Qualified Domain Name” (e.g.;  A computer known as the DNS Server is basically the Directory  Assistance (“411”) service which keeps track of and links the matching records (host/domain names to IP addresses)  to provide the requested IP information.  So, might become and computercoach 192.123.456.789, and so forth.

As I said above, prior to the recent explosion of high-speed connections and RDPs, your typical internet user was simply transparently connected to the internet directly by their ISP.  But there’s an obvious problem – there are literally billions of computer users on the planet, and only 4,228,250,625 addresses (that would every combination of 0 – 255 in four discreet segments of the address).  And this doesn’t begin to include every router, switch and server on the internet that also handles IP traffic, and must therefore be assigned its very own IP address!

So how does the Internet cope with this problem?  By assigning both public and private addresses.  Just as the public telephone number for IBM might be (212) 123-4567, although there could be possibly thousands of extensions that look just like individual telephone numbers in the private offices in the buildings, the internet has a public IP address, behind which that address is split through equipment like routers, switches and hubs into many, many private IP addresses (this is sometimes called an “intranet”) which are blocked from public view, just like those office phone extensions.  The beauty of this scheme is that, just like you can have a private telephone extension like “01” in literally millions of internal telephone systems, you can have the same plethora of private IP addresses in literally of millions of private internets.

There’s only one condition:  The public IP addresses must fall within a certain three blocks of all of the IP addresses available.  There are as follows:  First, through 10.255.255 (Subnet Mask of; therefore theoretically up to 16,777,215 Class “A” addresses – sufficient for a very large enterprise, like Fortune 500 global businesses, ISP).  Second, 172.16.01 through (Subnet Mask of; therefore theoretically up to 1,048,576 Class “B” addresses – such as large corporations, colleges, government agencies).  Finally, through (Subnet Mask of; therefore theoretically up to 65,536 (LINK) Class “C” addresses – such as consumer, office and retail establishments).  Classes D ( and E ( are private addresses which are allocated to organizations that require blocks of IP addresses.  And the range between Classes A and B (127.0 0 0 - are special “loopback” addresses which mean that you are connecting to your own machine.

So far as the public addresses, remember one more thing:  Not all addresses are being used at any given moment, certainly not all of the time.  Most ISP connections are managed through so-called Dynamic Host Configuration Protocol (“DHCP”) Servers, which manage a range of IP addresses and assign unused addresses from the range each time a user signs on or must be reassigned an IP if the lease expires and must be renewed.  Most cable and DSL modems and home routers provide DHCP, so that you don’t have to worry about conflicts between IP addresses within your network, as DHCP automatically assigns IP addresses to each device.

To find your computer’s IP address, you must first log on to your router’s software configuration (usually to view your IP addresses.  Your external (“Public”) IP address can also be found through such free services as ( and .  For more explanation about how subnets, subnet masks, gateways and hosts work, click on those definitions.


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