I have a network that serves three computers. There are no issues with this part of the existing network.

I have purchased a new Cisco SG350-28P 28-Port Gigabit PoE Managed Switch.

I need this new switch to have a VLAN for two computers.

These two computers should be able to connect to the Internet through this new switch that is connected to the network (so that the two computers can get Internet connection)

I need the new switch to have a network of network.

Is it possible?  Can anyone help me?

The problem is that when the switch has a default VLAN 1 and I get an address like from the existing network, then there is no issue getting Internet connection. The two computers get talking to each other as well as to the Internet. As soon as I make a new VLAN on the switch, the Internet connectivity is lost.

The current issue I am facing.

Losing Internet connectivity after creating a new VLAN for the two computers attached to this switch.

A heads up for those who are going to help. While this switch has an extensive CLI commands, but it is still limited. For examples, it does not support commands like, NAT or VTP.

Here is the relevant configuration of the Cisco SG350-28P when I do a SHOW RUN:

INTERFACE GigabitEthernet5
switchport mode access
switchport access vlan 5

. . .

The SHOW VLAN shows the following

Vlan    Name          Tagged Ports     Untagged Ports   Created by

---     ----          ------------     --------------   ---------

1        1                             gi1-28, Po1-8        DV

5        5                                                  S

I hope I was able to give the configuration that I was being asked about.

Network Diagram/Configuration

SG-350#sh vl
Created by: D-Default, S-Static, G-GVRP, R-Radius Assigned VLAN, V-Voice VLAN
Vlan       Name           Tagged Ports      UnTagged Ports      Created by
---- ----------------- ------------------ ------------------ ----------------
 1           1                            gi1-4,gi6-28,Po1-8        DV
 10         TEN                                  gi5                S

SG-350#sh ru
v2.5.0.92 / RTESLA2.5_930_364_107
CLI v1.0
file SSD indicator encrypted
ssd config
ssd file passphrase control unrestricted
no ssd file integrity control
ssd-control-end cb0a3fdb1f3a1af4e4430033719968c0
unit-type unit 1 network gi uplink none
vlan database
vlan 10
voice vlan oui-table add 0001e3 Siemens_AG_phone________
voice vlan oui-table add 00036b Cisco_phone_____________
voice vlan oui-table add 00096e Avaya___________________
voice vlan oui-table add 000fe2 H3C_Aolynk______________
voice vlan oui-table add 0060b9 Philips_and_NEC_AG_phone
voice vlan oui-table add 00d01e Pingtel_phone___________
voice vlan oui-table add 00d01e Pingtel_phone___________
voice vlan oui-table add 00e0bb 3Com_phone______________
bonjour interface range vlan 1
hostname SG-350
username CES password encrypted bb0f32698c29ea839a69da69fdb810b7005c219a 
privilege 15
clock summer-time EDT recurring usa
interface vlan 10
name TEN
interface GigabitEthernet5
 switchport access vlan 10
  • Please edit your question to include enough information to answer the question. For example, we need a good network description or diagram, the network device models, and the network device configurations. We cannot guess where you may have gone wrong.
    – Ron Maupin
    Mar 2, 2020 at 16:32
  • Also, network classes are dead (please let them rest in peace), killed in 1993 (two years before the commercial Internet!) by RFCs 1517, 1518, and 1519, which defined CIDR (Classless Inter-Domain Routing). Modern networking does not use network classes.
    – Ron Maupin
    Mar 2, 2020 at 16:34
  • I have edited the question to make it more focused and have followed your piece of advice, but I still see my question as closed. Could you re-open it for me, please? Mar 2, 2020 at 17:32
  • You still have not included configurations.
    – Ron Maupin
    Mar 2, 2020 at 17:33
  • 1
    You still have not included the configurations. Also, you will not be able to use VLANs that cross the unmanaged switch because you cannot configure VLANs and a trunk on it You must trunk the VLANs back to the router because you must route traffic between the VLANs. The VLANs should each have a separate network.
    – Ron Maupin
    Mar 2, 2020 at 18:51

2 Answers 2


You have some serious design problems. You must trunk both VLANs to the router, that means you will have a virtual interface for each VLAN, and you will need separate network addressing for each VLAN.

You cannot trunk through the unmanaged switch because it cannot be configured to trunk. You will need to move the connection from the unmanaged switch to the managed switch, and you will need to set up trunk configuration on both the managed switch and the router. The two VLANs will have separate addressing, so you either break the existing network into multiple networks, or you add a different network for new VLANs.

The router will need a virtual interface for each VLAN to be trunked across the link to the managed switch.

  • Thank you Ron for your input. Ron, according to your suggestion, "You must trunk both VLANs to the router", I do not have a router and I just have one VLAN, and that is VLAN 10. When I create VLAN 10, the PC5 and PC6, that previously did have Internet connectivity, lose their connection to the Internet. Mar 3, 2020 at 14:10
  • You have two VLANs: VLAN 1 and VLAN 10. Having multiple VLANs on a switch is like separating the switch interfaces to different, unconnected, physical switches. You must use a router between VLANs. If you do not have a router, you will not be able to contact other networks, including the Internet. Routers route packets between networks. Each VLAN is its own network, and it should have its own network addressing. Isn't there anyone else in your company that understands networking better?
    – Ron Maupin
    Mar 3, 2020 at 14:40
  • My mistake. I agree, I have two VLANs. I am taking the VLAN 10 now out of picture. Now, I only have VLAN 1 and that means I do have full Internet connectivity. My two new PCs can also talk to each other. The only requirement now is to make this VLAN a network of Can that be done, without losing Internet connectivity. Please note that the old three PCs do not have a requirement and are not supposed to talk to the two new PCs. To answer your question if there is anyone else who understands networking better, regretfully, there is no one else who knows better networking. Mar 3, 2020 at 16:17
  • As I have explained, you need to have two VLANs, each one on its own network, and you must trunk those VLANs from the managed switch to the router, and they must both be configured on the switch and the router. You cannot have an unmanaged switch between the managed switch and the router. Your existing network design is not going to work for this.
    – Ron Maupin
    Mar 3, 2020 at 16:20
  • The new managed switch is a layer-3 switch. Do you think that is going to make things easy and maybe we do not require a router because of that layer-3 capability in the new managed switch? Mar 3, 2020 at 19:27

If you want two subnets, and, you must have a router (or layer 3 switch) to communicate between them.

But that begs the question: Do you really need a separate subnet ( Can you put the new devices all on the same subnet, with the default VLAN 1?

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