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In our company we have a CISCO 2811 Switch accompanied by an HP1920 Switch. Our current configuration consists of 2 VLANS:

  1. VLAN-2 that has no DHCP in IP range 192.168.0.0/24
  2. VLAN-5 That contains DHCP in IP range 192.168.1.0/24

AFAIK VLAN stands for virtual LAN meaning that can share the same Ethernet cable.

So once I plug in a device, I need to place manually the IP so it can be connected to the Internet. If I just plug in the device to the network it won't be able to connect to the Internet because it will fail to fetch an IP.

So I wonder why the device connected to Ethernet is unable to fetch an IP from VLAN-5 but instead I need to configure it with an IP from VLAN-5?

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  • Apparently, your DHCP service doesn't work. You need to add a LOT of information about network to your question in order to enable an answer. Please add the (sanitized) configurations of the switches, the router in between and details to the DHCP server (which scopes are configured, how it is attached to the network and which DHCP relays are used).
    – Zac67 ♦
    Jun 12, 2020 at 12:16

1 Answer 1

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VLANs isolate two logical networks from each others.

In such configuration each port on the switches is configured to allow access to VLAN 2 OR VLAN 5 (except the ports that connect the switch together and connect the router).

So :

  • if the port you connect to is configured for VLAN 5, you will get an IP by DHCP

  • if it is configured for VLAN 2, since there's no DHCP in this VLAN you have to configure a static IP in the network associated with VLAN 2, i.e. 192.168.0.0/24

  • if it is configured for VLAN 2 and you configure a static IP in the network associated with VLAN 5 i.e. 192.168.1.0/24, then you won't have network connectivity since you are in the wrong VLAN.

This assuming the configuration on the VLANs are correctly done.

VLANs make separate network on the same physical switches. This is done for performance, security and management reasons. What allow communication between VLANs and from VLANs to remote network (such as another company office or the Internet) is the router(s).

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  • So each ethernet plug if configured for different VLANS right?
    – Dimitrios Desyllas
    Jun 12, 2020 at 9:51
  • Yes each port has a configuration to tell him to which VLAN the traffic on this port belong to. (VLAN capable switches usually have a default configuration such as each port belong to VLAN1, any other configuration is made by the network administrators)
    – JFL
    Jun 12, 2020 at 10:08

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