PxPixel
Explaining the IP Subnet Mask by Phil Hippensteel - AvNetwork.com

Explaining the IP Subnet Mask by Phil Hippensteel

Author:
Publish date:

Dear Professor Phil:
What is the purpose of the IP subnet mask and how does it work?
Scott, Charlotte, NC


Hello Scott.
The answer to this question has two parts. The first involves the interpretation of the IP address. IP addresses are 32 bits (four bytes) long. Devices interpret the address as two parts: . Consider the address 172.176.18.3. In the original class system, the first two bytes designate the network because the address starts with 172. Therefore, the computer’s network is 172.176.0.0 (zeroes are inserted for convenience.) We would say that the computer’s individual address is 18.3 within the network. The class system allows the boundary between the parts of the address to be at 8, 16 or 24 bits. As the Internet got larger, that became a problem. So the subnet mask was created.

If the mask 255.255.0.0 is used with the 172.176.18.3, the interpretation of the address remains the same as in the class system. Because 255.255.0.0 in binary is 11111111 11111111 00000000 00000000, devices use the positions in the binary representation of the address matching the ones as part of the network address and positions matching the zeroes as the designation of the individual computer in the network. They call this a 16-bit mask (meaning 16 ones). However, if the mask 255.255.255.0 were used, the binary representation of the mask has 24 ones: 11111111 11111111 11111111 00000000. Now the address 172.176.18.3 is interpreted as computer .3 on network 172.176.18.0. It is entirely possible and, in fact, quite common to use masks with 25 or 26 bits. In theory the mask can be almost any length. The critical factor is that all computers on a subnetwork must use the same mask. This is because the mask determines the size of the network and all devices need to agree on that size.

*Author Note: I think the binary strings (24 bits) should appear on the same line for clarity. One possibility in the layout is to center on a separate line.

Phil Hippensteel is a professor of Information Systems at Penn State Harrisburg. If you have a question for Professor Phil, email us at AVTintern@nbmedia.com.

Related

Explaining IP Network Connections by Phil Hippensteel

Dear Professor Phil, What happens when a device connects to an IP network? Simon, West Chester, PA Simon, you’ve asked a question about something that all of initiate on our computers every day but rarely contemplate exactly what it requires. As soon as there is a physical connection to a network switch, your netw

Understanding IPTV by Phil Hippensteel

Dear Professor Phil There is something that confuses me.  I know that changing channels in an IP video network takes longer because of a fact related to IP addressing, but I not sure of the details.  Also, why do these changes happen quickly in some cases but much more slowly in other cases? Kevin, Houston, TX  

Will the Change from IP v4 to IP v6 Impact AV Pros? by Phil Hippensteel

Dear Professor Phil, I’ve heard a lot about the change from IP version four to IP version six.  Is it really a big deal to AV professionals? Ben, Atlanta, GA Yes, it is a really big deal. Many people think that the significant issue is the change in the size of the IP address. It’s changing from four bytes to six

Digital Video and IP Video are Not the Same by Phil Hippensteel

Dear Professor Phil, I continually hear people talking about digital video and IP video as if they are the identical.  Are they the same? Sam,  Hershey, PA Sam, They are not, but they are closely related terms. Consequently, you are correct that many people who are very familiar with analog video seem to confuse

Why Are Some Codecs Implemented in Software? by Phil Hippensteel

Dear Professor Phil: I understand that some video codecs are hardware devices and some are implemented in software.  Should I care about the difference, and is there a trend relative to this difference? Lorie, Everett, WA Hello Lorie, You are correct that video and audio codecs are implemented in both hardwar

What causes “tiling” in video? by Phil Hippensteel

Dear Professor Phil: What causes “tiling” in video, particularly when watching HDTV? Jose, Tampa, Fl Hello Jose, To understand what causes tiling, we must first understand certain aspects of MPEG compression. This is the compression method used by nearly all modern video systems. The camera typically records 30

A Simple Way to Explain MPEG Compression by Professor Phil

Dear Professor Phil: I’ve picked up several computer books at trade shows and have read wiki’s about mpeg compression.  The explanations always seem to involve complex mathematics.  Is there a simple way to explain how it works? Sam, Hershey, PA The algorithms used in mpeg compression are indeed what most of us