Just wanted to ask some questions regarding on our project. I have a layer 3 switch having vlan10 - 120, 4 ports per vlan and assigned class C IP's, however due to lack of IP addresses it can only accommodate up to vlan70 so I decided to use class A IP's on vlan80-120 but it's not working, need your help guys on how to make this thing work. Thanks!

  • 1
    You really really need to give MUCH more information (and to forget about Class A and B and C and D and E IPs, those have been of no interest since the 1990s).
    – Law29
    Sep 28, 2016 at 7:42
  • Okay I'm sorry about that, let me make this clear. I have a layer3 switch having vlan10 to vlan120 (12 vlans) I already configured public IP addresses on vlan10 to 70 on our core switch and private IP addresses on vlan80 to 120 (different networks). any suggestions on how to access internet using private IP addresses (vlan 80-120)
    – Paolo
    Sep 28, 2016 at 9:07
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    You need to configure NAT on your switch to translate the private addresses to a public address. If you're looking for configuration help, please post a simple diagram of your network, along with your switch model and current configuration file.
    – Ron Trunk
    Sep 28, 2016 at 10:11
  • Wait a minute... you have 7 vlans, each using a /24 of public ip addresses but only 4 ports in each vlan? If each port connects to a single host then this sounds like a HUGE waste of public address space and you would better subnet your /24s into smaller subnets, then you'll have enough to use a public subnet on all your vlans.
    – hertitu
    Sep 28, 2016 at 11:41
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    Need your switch type and configuration in order to help.
    – Ron Trunk
    Sep 28, 2016 at 13:45

2 Answers 2


You need to use NAT to translate the private IP addresses to public addresses in order to route traffic from private addressing to the public Internet. You will need to set aside some public address space to be used for the public side of the NAT. Unfortunately, it is unlikely that you will be able to use NAT on a switch; you will probably need to use a router for that.

As something of an ISP, you really should not use RFC 1918 address space for your private addressing. RFC 6598, IANA-Reserved IPv4 Prefix for Shared Address Space, allocates the address block for ISPs to use as shared address space in order to not conflict with the RFC 1918 address space used by the customers.


Ok so I'll go and assume you mean you are not able to access the one subnet from the other. Since I don't know what switch you are using I'll only be able to guess the following: You have no routing enabled ..?

On Cisco you will need to add SVIs in those VLANsand assign them an IP-Address. After that you will need to activate routing with ip route and you're golden.

Not much more we can help here without some proper infos

  • If the question isn't clear then please don't make assumptions but ask questions in the comments. In fact Paolo already did clarify the question there before you answered.
    – hertitu
    Sep 28, 2016 at 11:39

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