cat /proc/sys/net/ipv4/ip_forward
0 - not a router
1 - is a router

MTU
  maximum transmission unit (2-nd level)
  1500 bytes (if no Jumbo Frames)
  1492 bytes (PPPOE)
  min - 576 (to work with IPv4)
  min - 1280 (... IPv6)
  ... MF flag - more fragments
  Detecting optimal MTU via probing:
    sending big packet with DF (don't fragment) flag

MSS
    maximum segment size (at TCP options)


ARP table (cache) matching MAC to IP addresses
ARP protocol - 
  Who has 192.168.0.1
  reply - MAC addr

arp -n
  show ARP cache


ICMP protocol
  ping (Type 8 Echo)


Kernell params:

sysctl -a | grep net.ipv4

# switched off packet forwarding
net.ipv4.conf.all.forwarding=0

sysctl net.ipv4.conf.all.forwarding=0
cat /proc/sys/net/ipv4/ip_forwarding
1


TCP/IP handshake
-> SYN <num>
<- SYN,ACK <num+1>
-> ACK


FIREWALL:

netfilter - system name
    Stateful(SPI - stateful packet inspection)

Routers:

SNAT
DNAT

ConTracking - connection tracking (for IPv4 limitations overcoming).