What is Cisco VTY?

vty stands for Virtual Teletype. What is Teletype?

The teletype, or teleprinter, is a device used for communicating text over telegraph lines, public switched telephone network, Telex, radio, or satellite links.

Wikipedia explanation of teletype


This means vty is essentially like a virtual computer screen plugged into the router that we can remotely access.

Both SSH and Telnet use this virtual monitor to let you see the router/switch.

The command

line vty 0 4

Configures 5 of these virtual teletypes (vty’s) for us to use. Can think of it having 5 monitors connected to the router. When you SSH to it, you are claiming one of these monitors. Cisco devices support up to a maximum of 16. 0-15

List of Default IPv4, IPv6, and MAC Multicast Addresses

Here is a list of common Multicast addresses.

Mac Addresses

CDP (Sends messages to)01:00:0C:CC:CC:CCMore info
MAC address Multicast Addresses


OSPF – Neighbor Discovery (All Routers)
OSPF – DR/BDR224.0.0.6More info
IPv4 Multicast Addresses


All IPv6 DevicesFF02::1
All IPv6 RoutersFF02::2
Router Solicitation?FF01::1?
IPv4 Multicast Addresses

UDM Pro Error Changing WAN IP Addresses

There appears to be a bug on the UDM Pro that you can encounter while trying to update your WAN IP addresses. The error was similar to “Can’t change IP Address “PublicIP” used in Default Network”


It appears that the issue stems from the Internet Source IP being used in the LAN Network settings.

The way to work around this is to disable the Internet Source IP. However, this is greyed out which keeps us from making any changes. We can however use the Chrome Developer tools to get around this restriction.

  • Enable the Legacy Interface. UniFi Network Settings -> System -> Legacy Interface
  • Go to Settings -> Networks -> Edit (Select Default Network)
  • Open up the Dev tools with Ctrl + Shift + i and select Console
  • Paste the following in and hit enter
$$('[disabled]').forEach( a => a.disabled=false )
Enable Internet Source IP on UDM Pro
  • Find “Internet Source IP”, Disable and Save!

Swap back to the new user interface and go change the WAN IP address.

Setup DHCP server on Linux

Install dhcp server software

sudo apt install isc-dhcp-server

Edit the following config file and set the networking interface it should use. In this case enp60s0

sudo vi /etc/default/isc-dhcp-server

Example line to change


Now edit the dhcpd.con file

sudo vi /etc/dhcp/dhcpd.conf 

Add the following in. Change the addresses and settings as needed.

subnet netmask {
   option domain-name-servers ns1.internal.example.org;
   option domain-name "internal.example.org";
   option subnet-mask;
   option routers;
   option broadcast-address;
   default-lease-time 600;
   max-lease-time 7200;

Set a static ip on the computer that’ll be acting as the dhcp server. You can set it as the gateway if it is the gateway.

Allow dhcp through the firewall

sudo ufw allow  67/udp
sudo ufw reload
Restart the service and connect a client.

sudo systemctl restart isc-dhcp-server

More info.

You can look at dhcp leases with the following command

tail -f /var/lib/dhcp/dhcpd.leases