add ipv4 addressing part 2 notes

contents/ipv4_addressing/README.md

@@ -5,6 +5,7 @@
 * [IPv4 Address](#ipv4-address)
 * [Decimal and Binary](#decimal-and-binary)
 * [IPv4 Address Classes](#ipv4-address-classes)
+* [IPv4 Address Configuration](#ipv4-address-configuration)
 
 
 ## IPv4 Address
@@ -34,20 +35,21 @@
 
 | Class | First octet | First octet numeric range |
 | :-: | :-: | :-: |
-| **A** | 0xxxxxxx | 0-127 |
+| **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 end is usually considered `126` because `127` first octet is used to test the **network stack (l3)** on the local device
+* `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 | /8 |
+| **A** | 0xxxxxxx | 0-127 (1-126)| /8 |
 | **B** | 10xxxxxx | 128-191 | /16 |
 | **C** | 110xxxxx | 192-223 | /24 |
 
@@ -61,6 +63,41 @@
 | **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`
@@ -76,3 +113,90 @@
 * 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](image.png)
+
+### Show interfaces status and configured IP addresses
+```log
+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
+
+```log
+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
+```log
+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
+```log
+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)#
+```