Basics of networking : Model,Router,firewall,Switch, wireless access points



Router:

 Layer 3 device of OSI model, deals with communication via IP address, Private Network, public network etc. routing protocols OSPF, etc are used, with concepts like routing tables.








DNS:

Domain Name server


DHCP:

Dynamic Host Configuration Protocol

========================================================================
NETWORKS , SUBNETWORKS and ROUTE

                                   ROUTING TABLE 

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default             gateway             0.0.0.0         UG    100    0        0 em2
15.15.0.0       10.10.4.99      255.255.255.0   UG     0      0        0 em3


ESSENCE of routing table :
Which is the next HOP/Router for the current packet.

How it decides ?

it looks at current packet's destination address, say 15.15.0.1, it search for all Destination IP and Subnet, its does AND Operation of "Destination" and  "Genmask" to see, if it matches the route/row in routing table.
Once it finds the match, then it looks at "gateway" IP and forwards that packet to that "gateway".

What is Gateway ?

Gateway can be next attached/pingable routers, any pingable/reachable network/VM.
though which we can reach the packet's destination IP address.


In my case, I had to add below in .80 machine to reach 15.15.0.0 network's IPS
15.15.0.0       10.10.4.99      255.255.255.0   UG     0      0        0 em3





============NETSTAT=============

On .80 machine

tcp6       0      0 10.10.4.80:44016        15.15.0.1:3083          ESTABLISHED

Here local port 10.10.4.80:44016 is connected with remote 15.15.0.1:3083

On .99 machine

tcp6       0      0 15.15.0.1:3083          10.10.4.80:44016        ESTABLISHED

tcp6       0      0 15.15.0.1:3083          :::*                    LISTEN

and is listening from any connection from any ip and port from outside, so its a server socket on .99

===================TCP DUMP===========================================

tcpdump -vv -X -i em3 'port 44016' -w dump.log

=======================================================================


Segmentation is way to keep the virtual network separate.
Like 
 VLAN, VXLAN,GRE, OpenFlow Rules, Network Namespace.


Subnet provide layer3 Addressing scope for network.

DHCP Software is implmented via dnsmasq service of linux.
    










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