Layer 3 and IP
- IPv4 is the main protocol at Layer 3 (Network layer)
- Every router interface needs an IP address from its connected network
- IP header contains:
- Source IP (4 bytes)
- Destination IP (4 bytes)
- Plus other fields (e.g., TTL, Protocol, Checksum)
IP Broadcast
- Example:
192.168.1.255/24is a broadcast address - When a computer sends a packet to this IP, it:
- Encapsulates it into an Ethernet frame
- Uses destination MAC address:
FFFF.FFFF.FFFF(broadcast)
Binary Conversion
Binary to Decimal
- Binary
11000000=192 - Bit positions:
128 64 32 16 8 4 2 1→ sum active bits
Decimal to Binary
- Example: Convert 255 → write decreasing powers of 2
- Helpful decimal values to remember:
128,192,224,240— help quickly identify subnet masks- Binary
1111= decimal15
Number Systems
| Base | Description |
|---|---|
| Binary | Base-2 → powers of 2 |
| Decimal | Base-10 → powers of 10 |
| Hexadecimal | Base-16 → powers of 16 |
IPv4 Address Classes
| Class | Binary Start | Range | Notes |
|---|---|---|---|
| A | 0xxxxxxx | 1–126 | 0.0.0.0 reserved, 127.x.x.x is loopback |
| B | 10xxxxxx | 128–191 | – |
| C | 110xxxxx | 192–223 | – |
| D | 1110xxxx | 224–239 | Multicast addresses |
| E | 1111xxxx | 240–255 | Experimental |
- Class is determined by the first octet
- Multicast ≠ unicast or broadcast
127.0.0.0/8= loopback range → used to test TCP/IP stack
Network vs Broadcast Addresses
| Type | Host Bits | Example | Assignable? |
|---|---|---|---|
| Network Address | All 0s | 192.168.1.0 | ❌ |
| Broadcast Address | All 1s | 192.168.1.255 | ❌ |
Subnet Breakdown (Example: 192.168.1.0/24)
| Address | Meaning |
|---|---|
192.168.1.0 | Network Address |
192.168.1.1 | First usable host |
192.168.1.254 | Last usable host |
192.168.1.255 | Broadcast Address |
- Host bits: 8 →
2^8 = 256total addresses - Usable:
256 - 2 = 254hosts
Cisco Router IP Configuration
Show interface summary
R1# show ip interface brief // or: sh ip int br
| Field | Description |
|---|---|
| OK | Legacy (no longer relevant) |
| Method | How IP was assigned (manual/unset) |
| Status | Layer 1 state (shutdown?) |
| Protocol | Layer 2 state (cable plugged in?) |
Configure IP Address on Interface
R1# conf t
R1(config)# interface gigabitethernet 0/0 // shortcut: in g0/0
R1(config-if)# ip address 10.255.255.255 255.0.0.0
R1(config-if)# no shutdown
R1(config-if)# do show ip int br // to verify
- To switch interface from config-if mode: repeat
interface g0/xcommand
Interface Commands
R1# show interfaces g0/0 // detailed info
R1# show interfaces description // summary
R1(config-if)# description ## to SW1 ##
R1(config-if)# do show interfaces description
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Category: CCNA