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My company asked me to set up a network with 60 devices (all have MAC addresses and use IPs, but they are not all computers), there is a 3rd party company that will maintain the network. They are also responsible for their internet access.

I received an email from the ISP with our network info: the LAN IP space we have is 60.140.130.192/29 (255.255.255.248) with a gateway of 60.140.139.193. Using the ISP's information, I'm not sure if all 60 devices can be on the network.

Is it possible to have 60 hosts on a 255.255.255.248 network?

  • 10
    Consider using NAT or PAT – Mike Pennington Jun 7 '14 at 16:42
  • 2
    And consider security too. – Remi Letourneau Jun 9 '14 at 20:31
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 6 '17 at 18:39
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Are you absolutely positive that all 60 devices need to have their own public WAN IP address? The ISP is giving you the globally reachable IP addresses for internet connectivity. You can have well over 100 devices use just 1 WAN IP from your ISP. Using NAT (network address translation) the 60 (Inside - 192.168.1.x /24) devices can share the 1 WAN (outside) IP address.

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NAT/PAT can be used to solve your problem, 60 host on few IP Address. For your deployment and if you don't need 1:1 translation, that's mean, many address can be translated in many address AKA many-to-many translation this could be a solution.

For example, if you want to deploy such scenario, here the commands for a Cisco Device.

! internal subnet to match 
(config)# access-list 1 permit 192.168.0.0 0.0.0.255 
! Public Pool where the private will be translated 
(config) # ip nat pool POOL 60.140.130.193 60.140.130.198 netmask 255.255.255.248
!
! List 1 will be translated on POOL 
(config) # ip nat inside source list 1 pool POOL
!
! LAN Interface 
(config) # interface FastEthernet 0/1
(config-if) # ip nat inside
! WAN Interface
(config-if) # interface FastEthernet 0/0
(config-if) # ip nat outside
!

For the outside point of view, your 60 host will fit on the public subnet.

Suggestion : since public IPv4 are a limited resource, the best would be to NAT/PAT the 60 Host on only one IP and leave some free Public IPs, in case of need. Well this suggestion don't take in account the limitations of NAT/PAT, like how much host can be translate over one 1 Public IP.

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255.255.255.248 = 11111111.11111111.11111111.11111000

Taking the off bits which are 3. 2^3 = 8(-2), with one network being for identification and one for broadcast you have 6 hosts per subnet.

And your range is 60.140.130.193 - 60.140.130.198

If you want to fit 60 devices you need a /26 which is 255.255.255.192

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1

Lets start with your stated question:

Is it possible to have 60 hosts on a 255.255.255.248 network?

As you may have expected, the answer is no.

@null's answer to the question you asked is correct and I do not understand why it was downvoted. With a /29 you have (32-29=) 3 bits for your own local networking. That is 23 addresses. You want one of those as broadcast adress, one as network adress *1 and one for your gateway. That leaves you with 5 IPs which can communicate directly with the internet.

The keyword here is directly.

In a perfect world you would have more than enough IP addresses to give each device their own IP. And you would use on of them for a firewall which you would put between the gateway and your internal network.

Sadly it is not a perfect world.

Free IP v4 addresses are getting scarce. There are two ways around that:

  1. Do not use IP v4 but switch to IP v6
  2. Use an ugly hack called NAT/PAT.

The latter is probably the easiest way to get all your 60 devices connected to a local LAN with non-public IPs



*1: You might get away with not using the last. But it might break compatability with a lot of devices and it will give you headache when people assume that both .0 and .255 (or rather, in your case .192 and .199) are used in the traditional way.

*2Please make sure you use non public IPs. E.g. 172.16.130.x. See RFC1918 for more information on these IPs. And pick a range which is not commonly used. That will prevent problems later on when people start trying different NAT area's to one

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  • 2
    The answer to the stated question is to use NAT. Also, you are inventing new requirements when you introduced the discussion about directly; there's no such requirement in the question. OP can obviously figure out that he cannot put 60 hosts 1:1 on a /29. In the real world it's simply unwise if you do not mention NAT/PAT when responding to this question. Perhaps null's answer was downvoted thusly – Mike Pennington Jun 23 '14 at 11:44
  • The ".0 and .255" addresses (network/subnet ID and subnet mask) in this particular case would be .192 and .199 – Avery Abbott Jun 24 '14 at 2:46
  • Whoops. That will teach me to calculate them myself rather then taking the easy 'cut-and-paste' method from another post (and where the numbers had another meaning. nm the usable IPs). – Hennes Jun 24 '14 at 6:52
  • 3^2 = 9 , it wrong calculation of addresses!!! 2^3 is right. – mmv-ru Dec 8 '14 at 14:07
  • Good catch. Fixed 3^2 (9) to 2^3 (8). I left the 8-3 is 5. – Hennes Dec 8 '14 at 23:52

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