I want my clients to use a /19 subnet but want them divided along 4 /21 subnets:

  • /21 for servers, printers and so on
  • /21 for LAN clients
  • /21 for WLAN clients
  • /21 vor VPN clients

The subnets for servers, LAN and WLAN clients shall be served by DHCP running on a server with multiple VLAN interfaces in each segment. Also, a router is configured with VLAN interfaces in each segment. My concern is if I need to configure the interfaces each as /19 or /21. Or do I have something completely misconceptualized?

Edit in response to Ron Maupin:

  • 1 EdgeRouter PoE
  • 1 Cisco SG300-52
  • 1 TP-Link TL-SG2452
  • 2 TP-Link TL-SG3424P
  • about 2 dozens of TP-Link TL-SG3210 in different rooms
  • about a dozen Ubiquity access points spread over the floor

The EdgeRouter PoE can provide DHCP but not dynamic DNS, so there's a server doing both. A single /24 subnet got to small, thus I have to redesign the network during productive use. One department working with credit card information and stuff like that needs its own seperate network, thus they have their own VLAN. Currently, we have our regular office VLAN (default 1), a guest VLAN (20), a VLAN for WiFi (50) and the seperated VLAN (100). Our dedicated server serves DHCP and DNS for VLANs 1 and 50 but I want it to do the others, too.

  • Is this an actuall, practical problem? Looks extremely like homework which is explicitly off-topic here.
    – Zac67
    Commented Jun 29, 2017 at 11:06
  • @Zac67, yes, this is an actual, practical problem.
    – MasinAD
    Commented Jun 29, 2017 at 11:11
  • You are going to need to tell us more about the network and what are the network device models and configurations.
    – Ron Maupin
    Commented Jun 29, 2017 at 13:38
  • @RonMaupin, I added information to the original post.
    – MasinAD
    Commented Jun 29, 2017 at 14:16

1 Answer 1


To answer your specific question, each VAN interface has a /21 mask. Think of the VLAN interface as just another host address on each subnet.

By the way, assuming a Cisco router, you don't need to have multiple interfaces on your DHCP server. You can have just one and configure

ip helper-address x.x.x.x

on each VLAN interface. x.x.x.x is the address of your DHCP server.


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