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204 Test > Subnets and Debugging (w9-10) > Flashcards

Subnets and Debugging (w9-10) Flashcards

1
Q

How would you map IP address to MAC address

A

broadcast packets to find a host

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2
Q

How are 0’s omitted in IPv6 addresses?

A

:: means all 0’s ::: not applicable

:1: means 0’s preceed the 1

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3
Q

when the destination is not in the local network, where is the message forwarded to?

A

the router, and a sequence of routers define a route from src to dest

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4
Q

the network ID portion of an IPv4 address can go up to how many bits? whats the rest of the bits used for?

A

up to 31 bits, rest of bits is used for Host ID

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5
Q

192.168.2.0/24 what does the 24 mean?

A

the first 24 bits correspond to the network address

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6
Q

how would we check if two ip addresses were on the same network?

A

the only way is to convert both addresses to binary, then compare the network ID’s. if they are the same, then they are on the same network

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7
Q

what are the two ways to write out the subnet masks?

A

/16 means first 16 bits are mask,
255.255.0.0 means first 16 bits are mask
1111 1111 1111 1111 0000 0000 0000 0000 is same as above

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8
Q

what is special about all ones host id, and all zero’s host id?

A

all ones host id is the broadcast address, all zero’s is the network identifier, used in routing tables. neither host id address is allocated to nodes on the network

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9
Q

if a host recieves a packet which has their address as a destination, what does the host do? what if the packet marks the hosts address as the source?

A

receives the packet if destination matches, forwards it if packet source matches hosts address

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10
Q

what is the routers policy on forwarding packets, in terms of the packets source address?

A

routers always forward packets, irrespective of the source address

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11
Q

What does a routing table consist of? do only routers have routing tables?

A

all end hosts and routers have routing tables. routing table has list of network id’s, gateways to relay frames to, and which network interface to use

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12
Q

when is a defualt route used?

A

when no specific address of a next hop host is available in the routing table. default route is one step closer to the internet

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13
Q

what is a loopback address used for, and example and what is the range of this loopback?

A

used for troubleshooting, in range 127.0.0.0/8

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14
Q

in larger network, does the upstream router know complex details about the downstream routers?

A

no

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15
Q 
in a routing table given the following pre-fixes, order them accordingly. what does this mean for /0?
/16
/0
/24
/23
A 
/24
/23
/16
/0
/0 is the last entry tried, as more specific entries are chosen first
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16
Q

Commands used for identifying issues. Host config? DNS? Path Issues? other issues

A

Host: ip
DNS: dig
Path: ping, traceroute, tcpdump, wireshark
Other: wireshark, tcpdump

17
Q

what can ping command help with

A

check if host or hostname is reachable, returns latency (RTT) and loss information

18
Q

Which kind of ICMP packet does ping send? which kind does it expect in reply?

A

sends echo request, expects echo reply

19
Q

how is traceroute similar to ping?

A

sends sequence of routers on forward path, reports delay along each hop, returns router hostnames from dns

20
Q

when TTL (timetolive) reaches 1 which ICMP packet does the router send back to the source? when is the TTL decremented, and why is it implemented

A

sends a ICMP TIme Exceeded message, saying packet was discarded. TTL stops packets getting stuck in infinite loops

21
Q

How does traceroute get a reply from each router?

A

by increasing the TTL value of each packet, so each router ends up returning a packet, so traceroute can see the full picture

22
Q

how would you add an ip address to interface eth0.

how would you add an route entry to 10.0.0.0/24 using the gateway router 192.168.1.1?

A

ip address add dev eth0

ip route add 10.0.0.0/24 via 192.168.1.1

23
Q

how to capture tcpdump from interface eth0

A

tcpdump -i eth0

24
Q

Should network N1 be connected to two different routers? why is this potentially confusing? whats the alternative?

A

N1 could be connected to two different routers, but this means each host in N1 would have 3 paths in their router tables. (N1 paths, router 1 path and router 2 path). It would be easier to have one router connected. (hosts would now have only two paths, default via router, or N1) And then confine all the complications to only the router.

25
Q

What are some advantages /disadvantages with static routing?

A

advantage: - reliable
- very configerable
disadvantage - does not respond to topology change
- can become complex

26
Q

what the the two dynamic protocol types?

A

internal and external routing protocols

27
Q

out of distributed calculation and distributed database, which uses Dijkstra’s algorithm>

A

Distributed database.

28
Q

how is Open Shortest Path First (OSPF) run? what hapens if a link breaks

A

All routers tell all other routers about their links and costs of using those links (flooding). all routers then use Dijkstra’s algorithm to calculate optimal topology. if link is broken or changes cost, graph is updated, and algorithm is run from scratch

29
Q

what is a link state advertisement, where is this sent to?

A

it is the links and subnets a router has, sent to the all-routers multicast address.

30
Q

BGP vs OSPF

Border Gate Protocol

A

OSPF is used internally, where BGP is routing between organisations

31
Q

130.217.0.0/16 38022 | 681 is an example of? how is it leading to a network?

A

example of route announcement in BGP

IP 130.217.0.0/16 is reachable through AS 38022 then AS 9433

32
Q

if waikato is announced like this: 130.217.0.0/16 38022 | 681, and gets a neighbour AS1234, how is the announcement process executed, what does AS1234 end up looking like?

A

it is announced from bottom up, so AS1234 announces itself to waikato like this:192.107.171.0/24 681|1234 which ends up looking like this to the internet192.107.171.0/24 38022|681|1234

33
Q

What two kinds of relationships are there between two AS’s?

A

either Provider to Customer, with customer paying for reachability, or Peer to Peer, where peers provide reachability to each others customers

34
Q

what is a local preference integer value? who gets priority?

A

its a value assigned to received routes that specify the priority of the incoming route. typically higher value == higher preference. priority is usually given in this order: customers, peers then providers

35
Q

Routers that access BGP do not often have default addresses, how is this possible?

A

they have a route to every prefix advertised by every address

204 Test (7 decks)

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