Network layer functions:
1. defines the logical address based on the IP protocol;
2. connecting different types of media;
3. Select the best path through a data network.
IP packet format: 
Version (4): versions 4 and 6;
header length (4): header length IP packet;
priority service type (8): determines
the total length (16): the IP packet the total length;
identifier (16): each segment labeled packets;
flag (3): if a further segment;
segment offset (13): the need to use recombinant destination offset off;
the TTL (8): the life cycle: the number of data packets can be routed through;
protocol number (8): description upper layer protocol (TCP (6) or UDP (17));
header checksum (16): error checking;
source address (32): IP address, 32;
destination address (32): IP address, 32;
optional: expansion of content;
data.
: ICMP protocol and encapsulation 
described ARP protocol:
Communication between the LAN 1. The host: IP address and MAC address.
2. ARP: is to resolve a known IP address to a MAC address.
ps: cache table aging time is 120S.
Analytical procedure:
A.PC1 send data to the PC2, the cache does not view the MAC address of PC2;
B.PC1 ARP request message sending (broadcasting);
C. All hosts receive the ARP request message:
a.PC2 reply ARP reply (unicast) ;
. B discards other hosts.
有关的基本命令：
ARP spoofing principle: 
ARP *** Principle: