Shulou(Shulou.com)06/03 Report--

Functions of the network layer:

1. Define the logical address based on IP protocol.

two。 Connect different media types

3. Select the best path for data to pass through the network.

IP packet format:

Version (4): version 4 and version 6

Header length (4): header length of IP packet

Priority and type of service (8): decision

Total length (16): the total length of IP packets

Identifier (16): the tag of the packet in each segment

Flag (3): whether there are any subsequent segments

Segment offset (13): the break offset needed to reorganize to the destination

TTL (8): life cycle: how many routes a packet can go through

Protocol number (8): describes the upper layer protocol (TCP (6) or UDP (17))

First checksum (16): error check

Source address (32): IP address, 32-bit

Destination address (32): IP address, 32-bit

Optional: expand content

data.

ICMP protocol and encapsulation:

Introduction of ARP protocol:

1. Communication between hosts in a local area network: IP address and MAC address.

two。 Address resolution protocol: to resolve a known IP address to an MAC address.

Ps: the aging time of cache table is 120s.

Parsing process:

A.PC1 sends data to PC2 to check the MAC address of the cache without PC2

B.PC1 sends ARP request message (broadcast)

c. All hosts receive an ARP request message:

A.PC2 reply ARP reply (unicast)

b. Discard other hosts.

The relevant basic commands:

ARP spoofing principle:

ARP*** principle:

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