ccna-notes

IPv4 Addressing

IPv4 Address
Decimal and Binary
IPv4 Address Classes
IPv4 Address Configuration

IPv4 Address

32 bits / 4 bytes
We use dotted decimal instead of binary for ease of use
- 192.168.1.254 = 11000000.10101000.00000001.11111110
CIDR notation: /24 after the IP number indicates 24 out of the 32 bits in an IP address correspond to the network portion. Possible values are 1 > n > 32 where n is an integer. 🔥🔥🔥
- i.e 192.168.1.0/24

Decimal and Binary

Min/Max binary octet

Binary to Decimal

Decimal to Binary

IPv4 Address Classes

Class	First octet	First octet numeric range
A	0xxxxxxx	0-127 (1-126)
B	10xxxxxx	128-191
C	110xxxxx	192-223
D	1110xxxx	224-239
E	1111xxxx	240-255

A class range is considered to start at 1 because 0 first octet is used reserved
A class range is considered to end at 126 because 127 first octet is used to test the network stack (l3) on the local device
We will focus on A, B and C.
D are multicast addresses
E are reserved (experimental)

Class	First octet	First octet numeric range	Prefix Length
A	0xxxxxxx	0-127 (1-126)	/8
B	10xxxxxx	128-191	/16
C	110xxxxx	192-223	/24

Netmask

Class	Prefix Length	Netmask	Binary octets
A	/8	255.0.0.0	11111111 00000000 00000000 00000000
B	/16	255.255.0.0	11111111 11111111 00000000 00000000
C	/24	255.255.255.0	11111111 11111111 11111111 00000000

Maximum hosts per network

2^n - 2 🔥

where n = number of host bits

/24 (Class C)

192.168.1.0/24 — 192.168.1.255/24

Network portion: 24 bits
Host portion: 8 bits
Therefore 8 bits -> 2^8 = 256
BUT 192.168.1.0 is the network address (network id) and 192.168.1.255 is the broadcast address
Therefore max number of hosts is 2^8 - 2 = 254 🔥

/16 (Class B)

172.16.0.0/16 — 172.16.255.255/16

Network portion: 16 bits
Host portion: 16 bits
Therefore 16 bits -> 2^16 = 65536
BUT 172.16.0.0 is the network address (network id) and 172.16.255.255 is the broadcast address
Therefore max number of hosts is 2^16 - 2 = 65534 🔥

/8 (Class A)

10.0.0.0/8 — 10.255.255.255/8

Network portion: 8 bits
Host portion: 24 bits
Therefore 24 bits -> 2^24 = 16777216
BUT 10.0.0.0 is the network address (network id) and 10.255.255.255 is the broadcast address
Therefore max number of hosts is 2^24 - 2 = 16777214 🔥

Network address

192.168.1.0/24

/24 means 192.168.1 is the network portion and .0 is the host portion
Since the host portion is 0 it means it is the network address
it is the first address with a host portion of all 0s

Broadcast address

192.168.1.255/24

last address in the network
with /24 net mask it would be the last octet .255
cannot be assigned to a host

IPv4 Address Configuration

Consider the following network where the router interfaces are yet to be configured: Screenshot from Packet Tracer showing 3 LANs connected to R1 router

Show interfaces status and configured IP addresses

Router>enable
Router#show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol 
GigabitEthernet0/0     unassigned      YES unset  administratively down down 
GigabitEthernet0/1     unassigned      YES unset  administratively down down 
GigabitEthernet0/2     unassigned      YES unset  administratively down down 
Vlan1                  unassigned      YES unset  administratively down down
Router#

4 Interfaces (G0/0, G0/1, G0/2 and G0/3)
No IP addresses assigned
OK? column is a legacy field not used anymore.
Method: method by which the interface was assigned an IP address
Status: L1 Status of the Interface 🔥
- “Administratively down” : the interface has been disabled with the shotdown command. This is the default status of Cisco router interfaces.
- Cisco switch interfaces are NOT administratively down by default
Protocol: L2 Status of the interface 🔥
- Because the interfaces are DOWN at L1, L2 can’t operate 🔥

Configure interface IP

Router#config terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#interface gi
Router(config)#interface gigabitEthernet 0/0
Router(config-if)#ip address 15.255.255.254 255.0.0.0
Router(config-if)#no shutdown
Router(config-if)#
%LINK-5-CHANGED: Interface GigabitEthernet0/0, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up

Router(config-if)#

interface gigabitEthernet 0/0 command to enter gigabitEthernet 0/0 interface configuration
ip address 15.255.255.254 255.0.0.0 to configure the IP address, we need to manually enter the subnet mask :fire:
no shutdown because Cisco router interfaces had the shutdown command applied to them by default
%LINK-5-CHANGED: Interface GigabitEthernet0/0, changed state to up indicates the physical layer status of the interface (Status column)
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up indicates the L2 status of the interface (Protocol column)

We’ll do the same for the other 2 interfaces

Router(config-if)#interface gigabitEthernet 0/1
Router(config-if)#ip address 182.98.255.254 255.255.0.0
Router(config-if)#no shutdown

Router(config-if)#
%LINK-5-CHANGED: Interface GigabitEthernet0/1, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up

Router(config-if)#
Router(config-if)#interface gigabitEthernet 0/2
Router(config-if)#ip address 201.191.20.254 255.255.255.0
Router(config-if)#no shutdown

Router(config-if)#
%LINK-5-CHANGED: Interface GigabitEthernet0/2, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/2, changed state to up

Router(config-if)#

Now we can verify our interfaces are configured and working as expected

Router(config-if)#do show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol 
GigabitEthernet0/0     15.255.255.254  YES manual up                    up 
GigabitEthernet0/1     182.98.255.254  YES manual up                    up 
GigabitEthernet0/2     201.191.20.254  YES manual up                    up 
Vlan1                  unassigned      YES unset  administratively down down
Router(config-if)#