What happens after 255 users on network?

The title mostly described my question. The question mostly comes from curiosity.

So, to my understanding, the network will have a subnet, most likely default 255.255.255.0, and there can be 192.168.0.(1-255) internal IP addresses inside the subnet correct? Please correct me if I'm wrong.

But let's say that a 256th user wants to get on the network, would the firewall or router (whatever giving out DCHP leases) create an entirely new subnet automatically? How would one go about handling this issue?

And if the answer would to be to create two subnets from the beginning, how do you determine which users go on what subnet, and how can you have the two subnets communicate to one another.

Lets say a server is on 255.255.255.1, how can an IP under 255.255.255.0 retrieve some files from said server.

I'm learning networking as you can tell and I have a lot of questions :D

Thanks to all who help!
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More about 255 users network
  1. Sounds like a homework question you are wanting us to answer for you.

    I will say that 255.255.255.0 subnet does not have 255 users, it only has 254 because you cant use .0 or .255 thus you can only use .1-.254.

    I would suggest to read about subnetting and DHCP.
  2. Proxify said:
    The title mostly described my question. The question mostly comes from curiosity.

    So, to my understanding, the network will have a subnet, most likely default 255.255.255.0, and there can be 192.168.0.(1-255) internal IP addresses inside the subnet correct? Please correct me if I'm wrong.

    But let's say that a 256th user wants to get on the network, would the firewall or router (whatever giving out DCHP leases) create an entirely new subnet automatically? How would one go about handling this issue?

    And if the answer would to be to create two subnets from the beginning, how do you determine which users go on what subnet, and how can you have the two subnets communicate to one another.

    Lets say a server is on 255.255.255.1, how can an IP under 255.255.255.0 retrieve some files from said server.

    I'm learning networking as you can tell and I have a lot of questions :D

    Thanks to all who help!


    so actually, only 254 can get on the network- 1 is reserved for gateway, 1 is reserved for broadcast.

    when the 255th thing tries to get on, there are a few possibilities- one, DHCP will fail as there are no more IP addresses to give out, or you will start to see network weirdness as the 255th thing will now have IP conflicts since it has the same IP address as somthing else. it kinda depends on how the networking gear implements or handles this situation.
  3. DHCP can't give out what you don't tell it to hand out. So DHCP won't fail, and it won't hand out one that it has already given out (NO IP conflicts as a result of DHCP handing out the same IP twice). DHCP will simply stop handing out new IP addresses. It will continue to function, and will renew existing leases.
  4. You just set up a larger subnet in the first place. Nothing happens automatically.
  5. ss202sl said:
    DHCP can't give out what you don't tell it to hand out. So DHCP won't fail, and it won't hand out one that it has already given out (NO IP conflicts as a result of DHCP handing out the same IP twice). DHCP will simply stop handing out new IP addresses. It will continue to function, and will renew existing leases.


    Exactly, the only way you would have DHCP conflicts is if you had a mixture of manual IPs and auto-dhcp. For this example, say the DHCP server on the router handed out an address and then latter the computer that was manually set to that IP then booted up and tried to use that same IP.

    Even a "$20 special" router would not assign a duplicate IP address with everything set to auto-DHCP.
  6. bliq said:
    Proxify said:
    The title mostly described my question. The question mostly comes from curiosity.

    So, to my understanding, the network will have a subnet, most likely default 255.255.255.0, and there can be 192.168.0.(1-255) internal IP addresses inside the subnet correct? Please correct me if I'm wrong.

    But let's say that a 256th user wants to get on the network, would the firewall or router (whatever giving out DCHP leases) create an entirely new subnet automatically? How would one go about handling this issue?

    And if the answer would to be to create two subnets from the beginning, how do you determine which users go on what subnet, and how can you have the two subnets communicate to one another.

    Lets say a server is on 255.255.255.1, how can an IP under 255.255.255.0 retrieve some files from said server.

    I'm learning networking as you can tell and I have a lot of questions :D

    Thanks to all who help!


    so actually, only 254 can get on the network- 1 is reserved for gateway, 1 is reserved for broadcast.

    when the 255th thing tries to get on, there are a few possibilities- one, DHCP will fail as there are no more IP addresses to give out, or you will start to see network weirdness as the 255th thing will now have IP conflicts since it has the same IP address as somthing else. it kinda depends on how the networking gear implements or handles this situation.



    I'm not sure about any weirdness, the dhcp won't hand out an address that is still known to be leased. Any devices after the limit that pass authentication will merely show unknown network and a 169 address. Only weirdness that could arise is if a person were to circumvent dhcp and assign a static, or if there's another device handing out addresses oblivious to the original (afaik).

    One thing I can tell the OP, however, is that the router(consumer router? Business grade switch?) will almost certainly not create a new scope of ips, none that I have ever used at least. I don't believe it to be an impossibility, but it is the sort of thing that should not be automated. I am skeptical that many routers that have dhcp on them even allow for creating new scopes, but I guess I've only really worked with the cheapies.

    I don't think, however, that scopes of ips need to be on different subnets. Why would you have a server on 255.255.255.1 and devices on 255.255.255.0? Even in a WAN, you can all be on the same subnet, can't you?
  7. I also think you are confusing a subnet of 255.255.255.0 to and ip address of 192.168.0.XXX.
  8. ahnilated said:
    I also think you are confusing a subnet of 255.255.255.0 to and ip address of 192.168.0.XXX.


    That makes more sense, I was wondering about his subnet question lol.

    After 192.168.0.xxx gets filled, if a user has the hardware and knowhow, the new scope would be logically 192.168.1.xxx though when creating an internal scope you can assign it as you want, 1 is just the next number up. You could make it 6 or 88 or whatever.
    A larger scope can actually do some of this for you, with a 255.255.0.0 subnet dhcp will fill scopes through 192.168.xxx.xxx instead of limiting it to the last three digits. I've never done this so forgive me if my information is innacurate for this band of ips, but this kind of scope is the solution when you have thousands of devices and don't want to sit around making new ranges all day.
  9. First off, thanks for all the replies!

    But to comment on a few things...

    - This is not a homework assignment or anything related to school, I am simply curious and would enjoy having the knowledge in case it ever becomes helpful.

    - Someone mentioned that you can have more then 254 IPs per subnet? How would you go about doing this? Or did I misunderstand what was being said?

    - Someone else mentioned that the 3rd index (192.168.xxx.(1-254)) would be incremented. Would that automatically be done by DHCP?
  10. How many IP addresses are you looking for? There are tons of write ups on sub netting and IP addresses.
  11. You can have as many addresses (within reason) as you want in a subnet. For example 192.168.0.0/23 supports 510 devices.
  12. Proxify said:
    First off, thanks for all the replies!

    But to comment on a few things...

    - This is not a homework assignment or anything related to school, I am simply curious and would enjoy having the knowledge in case it ever becomes helpful.

    - Someone mentioned that you can have more then 254 IPs per subnet? How would you go about doing this? Or did I misunderstand what was being said?

    - Someone else mentioned that the 3rd index (192.168.xxx.(1-254)) would be incremented. Would that automatically be done by DHCP?
  13. ahnilated said:
    How many IP addresses are you looking for? There are tons of write ups on sub netting and IP addresses.


    Hypothetically anything above 254.... sooo let's say 500. Would the DHCP autoincrement after 254? So after 192.168.0.254 would it go to 192.168.1.1?
  14. Proxify said:
    ahnilated said:
    How many IP addresses are you looking for? There are tons of write ups on sub netting and IP addresses.


    Hypothetically anything above 254.... sooo let's say 500. Would the DHCP autoincrement after 254? So after 192.168.0.254 would it go to 192.168.1.1?


    If you used the subnet 192.168.0.0/23 DHCP would allocate (assuming that the address of the router is 192.168.0.1) 192.168.0.2 ... 192.168.0.254, 192.168.0.255, 192.168.1.0, 192.168.1.1 ... 192.168.1.254. That's a total of 509 addresses, plus the router, making 510 in all. Of course this depends upon you setting the DHCP server to allocate all those addresses. If you needed more you could use the subnet 192.168.0.0/22 to give a total of 1022 addresses. And so on.
  15. Whatever you subnet size is, subtract 3 ips, the high, low, and the gateway. Once you run out of IPs, you run out of IPs. That's it. Like running out of phone numbers. The only clean way around it is to increase the subnet size or to create a new subnet and place the new device on the new subnet. Then traffic needs to be routed between the subnets.

    Also, you do not just create a new subnet in the same broadcast domain, don't do that, bad things happen.
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