Having read: "Follow these simple steps to spot an IP address that is invalid: 1) Host without a subnet mask..." Doesn't absence of subnet mask mean that subnet mask will be implied by IP class? Or classful networks http://en.wikipedia.org/wiki/Classful_network are now impossible? Then what are the network classes for? to dump my poor brains

Hi, Thanks for the post. Please understand that subnet mask is used to determine what subnet an IP address belongs to. If we just know IP addess instead of subnet mask, we cannot determine what subnet it belongs to. As we know, not all IP addresses follow this standard subnet ranges for class A, B, C. For more information, you could refer to Planning Variable Length Subnet Masks (VLSM) Hope this helps. Miles Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.

Hi, thanks for answering. Should I understand that network classes concept (and implicit correlation between IP and its class, .e. network mask) is defunct and is neber currently used. Why, then, all handbooks are filled with dumb useless concepts and information?

> Should I understand that network classes concept (and implicit correlation between IP and its class, .e. network mask) is defunct and is neber currently used. No, it's not defunct. Classful uses direct related subnet masks for the IP range class, whereas classless does not follow the range class. THis came about when IP addresses started falling short in the early 90's. Therefore someone came up with the idea to break down a classful IP range into subranges, such as what subnetting does. This way you can take a range such as 192.168.1.1-192.168.1.254 with 254 useable IPs, and break it down into smaller subnets that become individual subnets instead of wasting an unneeded 254 IPs for a customer or network that only needs 20. > Why, then, all handbooks are filled with dumb useless concepts and information? They are not useless concepts and information. Technical engineering concepts are specific and to the point. If one doesn't understand them as presented, it can lead to frustration. Ace Fekay, MVP, MCT, MCITP EA, MCTS Windows 2008 & Exchange 2007, MCSE & MCSA 2003/2000, MCSA Messaging 2003, Microsoft Certified Trainer, Microsoft MVP - Directory Services. This posting is provided AS-IS with no warranties or guarantees and confers no rights.

So, having enetered, for ex., 194.11.11.11 as IP address does not require the entering of subnet mask if it is 255.255.255.0 (in NIC TCP/IP props o a computer)?

Yes, today's computers and probably just about any modern device running TCP/IP will require subnet mask information. As Ace eloquently described (he really does do a great job in his explanations...must be a teacher), there was a time long ago when IPs were not so scarce. IPs were simply broken into ranges (Classes). Subnet masks were not needed. Devices simply treated IPs according to the class they belonged to. Of course with the use of the subnet mask, we can now break up network ID into subnets. If you notice, Class A creates the range 1-127, B's range is 128-191, and C is 192-223. These numbers are not random. Convert them to binary and you will notice that any IP in the A range all share a common characteristic...The first bit in the first octect will be zero (0). All Class B addresses also share a common characteristic...their first bit is one (1) and the second bit is always (0). Class C addresses all share the first bit as one (1) and the second bit as (1). Visit: anITKB.com, an IT Knowledge Base.

So, legacy (ancient) devices do not require configuration of subnet mask (255.255.255.0) in order to participate in the modern local network 194.11.11.0/24? BTW I use in my PC 18-year old network card. How old should be device in order to facilitate configuration taks?

So, legacy (ancient) devices do not require configuration of subnet mask (255.255.255.0) in order to participate in the modern local network 194.11.11.0/24? BTW I use in my PC 18-year old network card. How old should be device in order to facilitate configuration taks? To enter an IP without a mask as they did in the caveman days would require an operating system that allowed to enter it without a mask from the same era. On modern day machines, if you enter for example, a Class C IP address, such as 192.168.1.10 and hit the Tab key, it will automatically assume a Class C full subnet, 255.255.255.0. Try it. And no, it will not automatically enter the gateway or anything else. :-)Ace Fekay, MVP, MCT, MCITP EA, MCTS Windows 2008 & Exchange 2007, MCSE & MCSA 2003/2000, MCSA Messaging 2003, Microsoft Certified Trainer, Microsoft MVP - Directory Services. This posting is provided AS-IS with no warranties or guarantees and confers no rights.

Other than network devices probably now located in museums or at the bottom of landfills, I can't think of an instance where I was able to configure a TCP/IP host without a subnet mask. Visit: anITKB.com, an IT Knowledge Base.

[...] As Ace eloquently described (he really does do a great job in his explanations...must be a teacher), [...] Thanks, Jorge. I try to come up with analogies to help explain concepts, but it's not always easy. :-)Ace Fekay, MVP, MCT, MCITP EA, MCTS Windows 2008 & Exchange 2007, MCSE & MCSA 2003/2000, MCSA Messaging 2003, Microsoft Certified Trainer, Microsoft MVP - Directory Services. This posting is provided AS-IS with no warranties or guarantees and confers no rights.