OSI Model CTF | NetSec

#01IP Protocol Layer

50 ptsOPEN▼

The Internet Protocol (IP) is responsible for logical addressing and routing packets between networks. At which OSI layer number does IP operate? Routers work at this same layer.

# Wireshark capture header:
Frame 1: 74 bytes on wire
Ethernet II, Src: aa:bb:cc:dd:ee:ff
Internet Protocol Version 4, Src: 10.0.0.1
Transmission Control Protocol, Src Port: 443

Remember: "All People Seem To Need Data Processing" — layers 7 down to 1.

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#02Transport Layer PDU

75 ptsOPEN▼

Each OSI layer wraps data in its own Protocol Data Unit (PDU). At the Transport layer (Layer 4), TCP wraps the data in a specific PDU. What is it called?

Layer	Protocol	PDU Name
7 — Application	HTTP, DNS, FTP	Data / Message
4 — Transport	TCP / UDP	???
3 — Network	IP, ICMP	Packet
2 — Data Link	Ethernet	Frame
1 — Physical	—	Bit

It sounds like a sports term. TCP adds a header to create this unit.

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#03MAC Address Layer

75 ptsOPEN▼

MAC (Media Access Control) addresses are hardware addresses burned into network interface cards. Switches use MAC address tables to forward frames. At which OSI layer do MAC addresses and switches operate? Enter the layer number.

# Ethernet frame structure:
[ Dest MAC | Src MAC | EtherType | Payload | FCS ]
# MAC = 48-bit hardware address
# Example: aa:bb:cc:dd:ee:ff
# Switches forward based on MAC table
# This is Layer ?

One layer above Physical. The PDU here is called a "Frame".

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#04HTTP Layer

100 ptsOPEN▼

HTTP, HTTPS, FTP, DNS, SMTP — these protocols form the foundation of internet services that users interact with directly. What is the OSI layer number where these application-level protocols operate?

# HTTP Request example:
GET /index.html HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0

# This operates at OSI Layer ?

It's the very top of the OSI model. The highest layer number.

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#05Encapsulation Order

150 ptsOPEN▼

When data travels down the OSI model from sender to receiver, each layer adds its own header — this process is called encapsulation. At the receiving end, headers are stripped off layer by layer. What is this reverse process called? (one word)

# Sending: Data → Segment → Packet → Frame → Bits
# Receiving: Bits → Frame → Packet → Segment → Data
Layer 7 app data
└─[L4 hdr] segment
└─[L3 hdr] packet
└─[L2 hdr] frame
└─ 01010... bits
# The removal of headers = ?

The opposite of "encapsulation". Think: de-capsulation or de-encap...

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