IPv4 addressing (2)

IP class (extra)

IPv4 addressing (1)#IPv4 class

• Class A address range is 0-127, but the usable range is 1-126, as 0 and 127 are reserved.

• Max hosts per network = (2^n) - 2, where n = no. of host bits.

• Network address + 1 = first usable address.

• Broadcast address - 1 = last usable address.

IPv4 addressing

Brief

• Goto privileged EXEC mode.

• R1#show ip interface brief (to confirm the status of each interface on the device & their IP addresses).

• Interface : lists the network interfaces on the device (eg: GigabitEthernet0/0).

• IP-Address : lists the IP address of each interface.

• OK? : legacy command which says whether the IP address is valid or not (modern devices won't let you assign invalid IP addresses).

• Method : indicates the method by which the interface was assigned an IP address ('unset' by default, 'manual' if you configure it).

• Status :

  • Layer1 status of the interface.

  • If the interface is enabled, cable is connected and other end of the cable is properly connected to another device, status displays 'up'.

  • Default status of Cisco router interfaces : 'administratively down', which means the interface has been disabled with 'shutdown' command.

  • Even when the interfaces are connected to switches, the interfaces remain 'administratively down' because the 'shutdown' command is applied to them by default.

  • Cisco switch interfaces are NOT administratively down by default (either up - when connected to a device, or down - when not connected).

• Protocol :

  • Layer2 status of the interface.

  • Because the interfaces are down at Layer1, Layer2 cannot operate. So all are 'down'.

  • 'down' in Status column and 'up' in Protocol column is NOT possible, although the reverse is possible.

• Once you configure and enable the interfaces, you should see 'up' in both Status and Protocol columns.

Configure

• Enter global configuration mode.

  • R1#conf t

• Goto 'interface config' mode, to configure the interface

  • R1(config)#interface gigabitethernet0/0 -> R1(config-if)#

  • shortform : R1(config)#in g0/0 -> R1(config-if)#

  • from here you can jump to another interface without coming back to global config mode.

• Set the IP address

  • R1(config-if)#ip address 10.255.255.254 255.0.0.0

  • If you don't know what to type next, use '?'

  • Type the subnet mask in dotted decimal instead of prefix.

• Enable the interface

  • R1(config-if)#no shutdown

  • Cisco router interfaces have 'shutdown' command applied to them by default.

• Check the brief

  • R1(config-if)#do show ip interface brief

  • 'do' is added to execute 'privileged EXEC' mode command from 'interface config mode'.

Extra 'show' commands

• R1#show interfaces (interface)

  • shows the detailed Layer1 and Layer2 info about the interface (and some Layer3 info as well).

  • can use this command on a Switch too.

• R1#show interfaces description

  • displays info like Status, Protocol from 'show ip interface brief' command along with some description, which helps us understand the purpose of the interface.

  • Configuring description :

    • Enter interface config mode.

    • R1(config-if)#decription ## to SW1 ##