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What is the default subnet mask of a Class A IP address?

  • 255.255.0.0 or /16
  • 255.255.255.0 or /24
  • 255.0.0.0 or /8
  • 255.255.255.255 or /32
  • I’m not sure, but I’m excited to learn more!

Correct Answer: 255.0.0.0 or /8

Explanation

Thanks again to Sean for submitting this one!

Class A networks are designed for very large organizations, with only the first octet being designated as the network identifier.

Further Reading

Visit the Daily Challenge category and discuss previous questions.

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2

I have seen that netmask used, but usually in routing tables/route summarisation. Not sure if I’ve ever seen it on a devices network config.

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Class A IP Range: 1.0.0.0 to 126.255.255.255
Default Subnet Mask: 255.0.0.0 or /8

Means: 8 bits are used for the network portion, and 24 bits for the host portion

  • Allows for over 16 million host addresses in each Class A network—massive!!!
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4

Curses. Wrong end.

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5

I am surprised to see how many picked the class C subnet answer.

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checks calendar it’s not Monday!

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7

Subnet masks and their numbers used to confound me until I learned about breaking it down to binary.

Write out the mask in binary, and where the 1’s are is the Network, and where the 0’s are are for the nodes!

11 Cheers for Binary!

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Tried to simply post this:

/16 /8 /24 O N

Spiceworks was having none of that though! LOL

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yayyy! I’m on a (short) roll of not second guessing myself. I almost did and then said “NO SIR, THAT’S IT! YOU KNOW THIS!” lol

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I did most of my learning figuring out stuff on my own. When I finally figured out sub-mask, I looked at it as “what network address will you let me look at?”. Anything with a 1 was forced and you could look at anything with a 0. With only that information, I wondered if you could give a subnet mask of 255.255.253.0 (11111111.11111111.11111101.00000000) to hide certain addresses. I never did get to try it out, and I’m assuming that applications won’t allow you assign a SM like that.

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That’s a fun thought experiment. I don’t see any technical reason why it wouldn’t work, but you’re right that a lot of gear would probably block you from doing that due to how much is lost in the way that we make it human-readable.

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Classful addressing is a really dated 90’s way specifying/describing networks.

Classless Inter-Domain Routing CIDR and interrelated VLSM are more succinct in describing network/prefix lengths.

CIDR is more for backbone (and some edge) routing and summarizing purposes. VLSM is how you slice and dice IP address range allocations of your choosing or provided by your ISP.

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13

I have not seen it with my own eyes, but I have heard of it being used by way of a remote user. They were having trouble accessing anything in our network despite being connected in through the VPN. We had configured and tested the VPN client on the laptop before issuing it for use so it was puzzling at first why it wasn’t working for him. I suspected that maybe there was an IP conflict between something on his local network and something on our local network so I directed him on how to open up a command prompt and run ipconfig. I had him read off his IP address but it sounded like neither it nor any other device on his end should have been conflicting with any of our local addresses. Then I had him read off his subnet mask…

It turns out that whoever set up the network provided by in his housing development (I think he said they were a bunch of condos) set the subnet mask to 255.0.0.0 so that it was claiming the entire 10.x.x.x address block for their (presumably relatively small) complex. :person_facepalming: Whoever set that local network up obviously had no clue what they were doing and all the problems that it could cause down the line. Because of that setup, he literally could not connect into resources on our network because we use a subset of the 10.x.x.x block but anything in the entire block was treated as if it was local subnet traffic and thus couldn’t get routed down the VPN tunnel accordingly.

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I’m pretty sure the way most gear is programed is that it stops looking if any bits are 1 once it comes across a 0 and just assumes all the other lesser significant bits are 0 as well. There’s no reason I’m aware of according to the IPv4 standards why you couldn’t have a subnet mask like 255.255.170.170. However, once we went with classless addressing and the notation it introduced, that pretty much would have put an end to it being able to use them effectively since you can’t really represent a non-contigous subnet mask using the slash notation.

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15

You mean you’ve never dealt with a /21 network before??

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16

I actually saw that once, it was in a university network with a really weird network split, they flipped a bit in the middle to allow some addresses to ‘leak’ through. I have never seen it anywhere else..ever..

Heres a bonus question: How many Class A networks can there be?

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Some routers (probably most/all higher spec’ed routers) will give higher preference to more specific routes .. so a /32 will get preference over a /28 which will be preferred to a /24 which is preferred over a /16 which is preferred over a /8, for example.

So even though there may be a default route for 10.0.0.0/8, a route for 10.14.0.0/16 will override it.

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It’s not that there was a default route for 10.0.0.0/8 on the complex’s router but rather that the laptop’s network subnet was defined as 10.0.0.0 (thanks to whatever was acting as a DHCP server at his complex).

Any custom routing (assuming it wouldn’t have simply been ignored due to those subnet settings) would have had to have been done on his laptop which was a problem as 1) the address for the VPN interface that we’d need to direct traffic to is also set via DHCP (which could make specifying the correct IP interface a bit problematic) and 2) the remote user wouldn’t have had the necessary administrative access to add a custom route in the first place.

While I could technically have added a static pool against his system’s MAC address to the DHCP server on our firewall (which is a bit of a PITA to configure compared to converting a given address lease to a reservation on a Windows DHCP server) to address the VPN interface addressing issue, we still would have run up against the problem of him not having administrative access … and no way am I handing out any sort of administrative access to a sales guy, doubly so for a remote one. :face_with_tongue:

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So, what did you do to finally ‘fix’ the issue?

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That all depends on what you consider to be ‘fixed’. In the interim, he’d just have to use other means available to him to connect to the Internet in order to get our VPN working properly (like, for example, use his cell phone as a wireless hotspot).

However, the longer-term ‘fix’ ended up being that, like a fair number of remote sales guys before him, he wound up quitting to go work somewhere else. With the exception of the managers and a few representatives that had pretty good sales territories to work with, the sales department is a bit of a revolving door when it comes to employees. I’m sure we’re not the only place like that either (as some will leave for other companies and later come back).

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