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Introducing Routers
Wednesday, November 18, 2015

Router

9:50 AM

A piece of hardware or software that forwards packets based on their destination IP address
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- Typical home routers are routers, switches, firewalls, a DHCP server and other features built together
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Meaning that
one side will have 1 port, and the other will have 4 or 5 ports
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The side with 4 or 5 ports will not use a external switch but will have each individual computer
connect together
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- First the router looks at the frame from whatever port it came in from
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- Routers use a Routing Table
- Routers don’t care the destination of the packet, they just queue the packet based on time received
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All IP-Enabled devices, including routers and
switches, use these
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168
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0 255
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255
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0
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0

192
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15
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201
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0 255
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254
...
0
...
0

98
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164
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0
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0

0
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0
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201
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1 98
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164
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- The reason you can't use a host ID of 0 is because it is reserved for routing tables
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0
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0
Default routs are used when frames are sent to one router and want to be send to another router
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Port numbers 0 1023 are called well known ports
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It's usually a number
well past 1023
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When it first receives the packet, it looks at the ports, it then
strips off the port data and sends up the data
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Routing Page 2

Network Address Translation
Tuesday, December 01, 2015

8:58 AM

NAT

Network Address Translation

PAT

Port Address Translation

Every router has a NAT
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Each device connected to the routers is assigned a LAN IP address and when it wants to connect to the
internet, the router assigns it a WAN IP address
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DNAT

Dynamic NAT

Dynamic NAT is very rarely used today, but has a limit of WAN addresses on the router, so only a limited
amount of devices can go out at once
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SNAT sends specific traffic to one internal IP address
DNAT has a limited pool of internet addresses to give to a number of internal devices
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Routing Page 4

Port Forwarding
Tuesday, December 01, 2015

9:18 AM

Port
Application of NAT that redirects a communication request from one address and port
Forwarding number combination to another while the packets are trans versing a network gateway
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You can use port scanners to find vulnerable
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If a particular server uses a lot of ports, you can use Port Range Forwarding instead of individually
entering each port
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For example, FTP
listens on port 21 and responds on port 20
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Holy fuck, counter strike uses a lot of ports
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You can use third party software to add features
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(CompTIA)
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To picture the internet, think of routers surrounded by computers connected to them
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In this situation, routers can go down at any given point,
so when routers are relaying data, they have to find a different route
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When routing, routers will use the route with the least amount of metrics (Hop Count)
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MTU

Maximum Transfer Unit (How much data it can hull)

Bandwidth The range of frequencies within a given band, in particular that used for transmitting a
signal
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(Satellites will be longer)

Ethernet has a MTU of 1500 bytes
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Each of these ways have different MTU, so when data is transferring and all of the sudden
they have to use a different transfer protocol, routers have to convert the data to which ever protocol
they are now using (chopping the data up to fit the MTU)
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- Link State: Basically pings (Link state Advertisements) other routers every once in a while, and
notifies when there is a change in the routing table
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Dynamic Routing Protocols can be split into two groups:
- Interior Gateway Protocol (IGP)
- Exterior Gateway Protocol (EGP): The idea that if you're a ISP, you control a lot of routers
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To leave an
Autonomous system, you need to use a EGP protocol
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When ISPs communicate to each other, they use a number system unique to
ISPs
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Dynamic routing protocols are either interior gateway protocol or exterior gateway protocol
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RIP's Maximum hop count is 15
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OSPF

Open Shortest Path First

- Uses IGP
- Uses Link State Protocol
OSPF works by you configuring your router for the protocol
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They start communicating about
what networks they are connected to
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Routing Page 10

BGP
Wednesday, December 02, 2015

BGP

9:03 AM

Border Gateway Protocol

You can break the internet into two different groups:
- LANS: Each with own subnet, most work here (email servers, Webservers…)
- Router Networks: Groups of highly connected routers owned by ISPs, often miles apart from each
other carrying most of the LAN data across the internet
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(Every router wanting every path to every LAN)
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All of these Autonomous systems have to use BGP to connect to each other across the world
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BGP is the primary protocol for the internet

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