IPv4 addressing operates in the 3rd layer of the OSI model stack - the Network layer. * Provides connectivity between end hosts on different networks (ie. outside of the LAN). * Provides logical addressing (IP addressing). * Provides path selection between source and destination. * Routes operate at Layer 3. * The length of IP addresses is 4 bytes (32 bits).  ## IPv4 Address Classes IPv4 addresses are divided into five different classes. The class of the IPv4 address is determined by the first octet of the IP address.  The non `x` values in the first octet indicate the first possible IP address for the given IPv4 address class. Setting all remaining `x` to 1 indicate the last possible IP address. Therefore, all addresses between the first and last possible addresses are available for use. * Only Classes A, B, and C can be assigned to devices. * IPv4 class D addresses are multicast addresses. * IPv4 class E addresses are Reserved (experimental purposes). * For class A IPv4 addresses, the first and last addresses are reserved. Therefore, the usable addresses are in the range 1 - 126. The 127 address range is reserved for Lookback addresses. * The address range is 127.0.0.0 - 127.255.255.255. * It is used to test the 'network stack' (OSI, TCP/IP model) on the local device. * When a host sends network traffic in the mentioned address range, the traffic is simply processed back up the network model stack.  ## Maximum Hosts per Network Every network contains a network and broadcast addresses that cannot be used. * Suppose we have the address `192.168.1.0/24` * The network address is `192.168.1.0/24` * The broadcast address is `192.168.1.255/24` * The first usable address is `192.168.1.1/24` * The last usable address is `192.168.1.254/24`  $ Maximum\ hosts\ per\ network = 2^n - 2, n = number\ of\ host\ bits $ ## Netmask (Notation used by Cisco to denote IP class) * Class A: /8 --> 255.0.0.0 * Class B: /16 --> 255.255.0.0 * Class C: /24 --> 255.255.255.0 ## Configure Router  * `administratively down (status)`: Interface has been disabled with the `shutdown` command * This is the default status for Cisco router interfaces * Cisco switch interfaces are not administratively down by default. * `Method`: Method by which the interface was assigned an IP address. * `Protocol`: Because the interfaces at layer 1 are down, layer 2 cannot operate. * You will never see an interface with a `down` status and `up` in the protocol column. Although, the reverse is possible. #### Assign IP and Subnet Mask to router interface ``` R1#configure terminal R1(config)#interface [interface] R1(config-if)#ip address 10.255.255.254 255.0.0.0 R1(config-if)#no shutdown ``` #### Other Commands `R1#show interfaces [interface]` * Shows information primarily about the layer 1 and 2 of the interface, but also some layer 3. `R1#show interfaces description` * Displays interfaces, status, protocol, and description. Descriptions are optional, but can be useful in identifying the purpose of each interface.