A good puzzle to solve which will clarify your networking concepts. Questions: 1> How and in which sequence Learning, Flooding and Forwarding will happen in Switch? 2> Find MAC and IP on every communication link shown in the figure?

1. Networking​ ​Example
Consider​ ​that​ ​in​ ​private​ ​network​ ​A,​ ​Your​ ​PC​ ​is​ ​connected​ ​to​ ​some​ ​local​ ​server​ ​having​ ​DNS​ ​and​ ​DHCP​ ​support,​ ​and​ ​your
browser​ ​initialing​ ​the​ ​request​ ​for​ ​www.google.com​ ​a​ ​very​ ​first​ ​time.
Traversal​ ​from​ ​your​ ​PC​ ​to​ ​the​ ​server​ ​in​ ​network-B​ ​where​ ​this​ ​website​ ​is​ ​hosted,
A​ ​-​ ​Part​ ​of​ ​the​ ​network:
When​ ​you​ ​search​ ​for​ ​www.google.com​ ​on​ ​browser,​ ​it​ ​ends​ ​up​ ​at​ ​https​ ​request.​ ​First​ ​thing​ ​that​ ​PC​ ​has​ ​to​ ​do​ ​is​ ​to​ ​resolve
the​ ​DNS​ ​-​ ​‘​www.google.com​’​ ​.​ ​​ ​Every​ ​PC​ ​has​ ​its​ ​own​ ​database​ ​to​ ​hold​ ​DNS​ ​<->​ ​IP​ ​address​ ​mapping​ ​for​ ​the​ ​famous
domain​ ​names.​ ​So,​ ​PC​ ​will​ ​look​ ​into​ ​its​ ​database​ ​to​ ​resolve​ ​this​ ​first​ ​but​ ​let’s​ ​assume​ ​that​ ​it​ ​won’t​ ​find​ ​the​ ​entry,​ ​so​ ​PC
will​ ​generate​ ​DNS​ ​query​ ​to​ ​the​ ​configured​ ​DNS​ ​server​ ​-​ ​having​ ​IP​ ​192.168.1.5​ ​(Considering​ ​that​ ​user​ ​has​ ​already
configured​ ​DNS​ ​server​ ​address).
PC​ ​will​ ​look​ ​into​ ​his​ ​ARP​ ​table​ ​for​ ​the​ ​DMAC​ ​of​ ​192.168.1.5​ ​but​ ​let’s​ ​assume​ ​that​ ​it​ ​won’t​ ​find​ ​the​ ​entry,​ ​so​ ​PC​ ​will
generate​ ​ARP​ ​request​ ​having:
SMAC-​ ​*00:01
DMAC-*FF:FF​ ​(All)
SIP-​ ​192.168.1.10
DIP-​ ​192.168.1.5
Switch0​ ​will​ ​receive​ ​it​ ​and​ ​broadcast​ ​it​ ​to​ ​all​ ​the​ ​other​ ​ports.​ ​Switch0​ ​will​ ​also​ ​add​ ​an​ ​entry​ ​in​ ​its​ ​MAC​ ​forwarding​ ​table,
as​ ​it​ ​has​ ​received​ ​the​ ​request​ ​from​ ​that​ ​MAC​ ​the​ ​first​ ​time.​ ​DNS​ ​server​ ​will​ ​receive​ ​this​ ​request​ ​and​ ​generate​ ​unicast
ARP​ ​response​ ​having:
SMAC-​ ​*00:03
DMAC-*00:01
SIP-​ ​192.168.1.5
DIP-​ ​192.168.1.10

2. Switch0​ ​will​ ​again​ ​add​ ​an​ ​entry​ ​in​ ​its​ ​MAC​ ​forwarding​ ​table​ ​and​ ​forward​ ​this​ ​packet​ ​to​ ​PC​ ​based​ ​on​ ​look​ ​up.​ ​Now,​ ​on
reply​ ​of​ ​ARP​ ​PC​ ​will​ ​update​ ​its​ ​ARP​ ​table​ ​and​ ​send​ ​DNS​ ​query​ ​to​ ​DNS​ ​server​ ​with:
UDP/TCP​ ​Port:​ ​53
SMAC-​ ​*00:01
DMAC-*00:03
SIP-​ ​192.168.1.10
DIP-​ ​192.168.1.5
DNS​ ​server​ ​will​ ​reply​ ​with​ ​destination​ ​server’s​ ​public​ ​IP​ ​-​ ​144.11.1.10.​ ​Where,
UDP/TCP​ ​Port:​ ​53
SMAC-​ ​*00:03
DMAC-*00:01
SIP-​ ​192.168.1.5
DIP-​ ​192.168.1.10
In​ ​last​ ​two​ ​traversals,​ ​switch0​ ​would​ ​easily​ ​able​ ​to​ ​forward​ ​as​ ​MACs​ ​are​ ​already​ ​learned.
Now​ ​on​ ​the​ ​basis​ ​of​ ​subnet​ ​mask,​ ​PC​ ​will​ ​identify​ ​that​ ​144.11.1.10​ ​is​ ​not​ ​in​ ​the​ ​same​ ​subnet,​ ​so​ ​PC​ ​will​ ​decide​ ​to​ ​send
https​ ​packet​ ​stream​ ​to​ ​the​ ​default​ ​gateway​ ​-​ ​192.168.1.1​ ​of​ ​the​ ​local​ ​network​ ​(Considering​ ​that​ ​user​ ​has​ ​already
configured​ ​Local​ ​gateway​ ​address).
PC​ ​will​ ​look​ ​into​ ​his​ ​ARP​ ​table​ ​for​ ​DMAC​ ​of​ ​192.168.1.1​ ​but​ ​assume​ ​that​ ​it​ ​won’t​ ​find​ ​it,​ ​so​ ​PC​ ​will​ ​generate​ ​ARP​ ​request
having,
SMAC-​ ​*00:01,
DMAC-*FF:FF​ ​(All)
SIP-​ ​192.168.1.10
DIP-​ ​192.168.1.1
Switch0​ ​will​ ​receive​ ​it​ ​and​ ​broadcast​ ​it​ ​to​ ​all​ ​the​ ​other​ ​ports.​ ​Router0​ ​will​ ​receive​ ​this​ ​request​ ​and​ ​generate​ ​unicast​ ​ARP
response​ ​having:
SMAC-​ ​*00:02
DMAC-*00:01
SIP-​ ​192.168.1.1
DIP-​ ​192.168.1.10
Now,​ ​Switch0​ ​will​ ​again​ ​add​ ​an​ ​entry​ ​in​ ​its​ ​MAC​ ​forwarding​ ​table​ ​and​ ​forward​ ​this​ ​packet​ ​to​ ​PC​ ​based​ ​on​ ​look​ ​up.​ ​PC​ ​will
update​ ​its​ ​ARP​ ​table​ ​and​ ​send​ ​https​ ​data​ ​packet​ ​to​ ​gateway​ ​router​ ​having:
SMAC-​ ​*00:01
DMAC-*00:02
SIP-​ ​192.168.1.10
DIP-​ ​144.11.1.10
Communication​ ​between​ ​A​ ​nad​ ​B​ ​part​ ​of​ ​the​ ​networks:
Now​ ​packet​ ​is​ ​out​ ​from​ ​PC’S​ ​local​ ​network,​ ​and​ ​it​ ​has​ ​to​ ​find​ ​a​ ​route​ ​to​ ​reach​ ​to​ ​the​ ​local​ ​network​ ​of​ ​the​ ​destination
server.​ ​Ideally,​ ​depends​ ​on​ ​the​ ​administrator’s​ ​configuration​ ​across​ ​the​ ​entire​ ​path​ ​there​ ​would​ ​be​ ​static​ ​and​ ​dynamic
routing​ ​protocols​ ​running​ ​in​ ​the​ ​network,​ ​on​ ​the​ ​basis​ ​of​ ​them​ ​multiple​ ​router​ ​lookups​ ​will​ ​occur​ ​and​ ​packet​ ​will​ ​find​ ​a

3. path​ ​from​ ​Router0​ ​to​ ​Router1.​ ​Remember​ ​that,​ ​while​ ​traversing​ ​through​ ​internet​ ​ARP​ ​and​ ​MAC​ ​forwarding​ ​lookups​ ​and
related​ ​operation​ ​will​ ​occur​ ​frequently​ ​as​ ​L2​ ​traversals​ ​are​ ​essential​ ​for​ ​​ ​L3.
Not​ ​going​ ​in​ ​further​ ​detail,​ ​when​ ​packet​ ​leaves​ ​Router0​ ​for​ ​the​ ​next​ ​route.​ ​The​ ​contents​ ​will​ ​be​ ​as​ ​shown​ ​below​ ​where
due​ ​to​ ​NAT​ ​private​ ​IP-​ ​192.168.1.10​ ​is​ ​replaced​ ​by​ ​public​ ​IP-​ ​11.1.1.75​ ​and​ ​entry​ ​for​ ​the​ ​same​ ​is​ ​also​ ​added​ ​in​ ​the​ ​NAT
mapping​ ​table​ ​of​ ​the​ ​router.
SMAC-​ ​*00:55
DMAC-*00:XX​ ​(Any*)
SIP-​ ​11.1.1.75
DIP-​ ​144.11.1.10
When​ ​packet​ ​will​ ​reach​ ​to​ ​Router1,​ ​the​ ​contents​ ​would​ ​be​ ​like,
SMAC-​ ​*00:XX​ ​(Any*)
DMAC-*00:A1
SIP-​ ​11.1.1.75
DIP-​ ​144.11.1.10
At​ ​Router1,​ ​there​ ​will​ ​be​ ​a​ ​NAT​ ​mapping​ ​between​ ​public​ ​IP-144.11.1.10​ ​and​ ​private​ ​IP-​ ​10.10.1.100,​ ​so​ ​the​ ​packet​ ​will​ ​be
modified​ ​accordingly​ ​to​ ​use​ ​private​ ​IP​ ​in​ ​the​ ​local​ ​network​ ​of​ ​destination​ ​server.
B​ ​-​ ​Part​ ​of​ ​the​ ​network:
Considering​ ​Router1​ ​as​ ​a​ ​default​ ​gateway​ ​for​ ​the​ ​private​ ​network​ ​of​ ​the​ ​server​ ​and​ ​we​ ​want​ ​to​ ​forward​ ​https​ ​request​ ​to
DIP:​ ​10.10.1.100.​ ​Router1​ ​will​ ​look​ ​into​ ​his​ ​ARP​ ​table​ ​for​ ​the​ ​DMAC​ ​of​ ​10.10.1.100​ ​but​ ​consider​ ​it​ ​won’t​ ​find​ ​it.
Router1​ ​will​ ​generate​ ​ARP​ ​request​ ​having:
SMAC-​ ​*00:F1
DMAC-*FF:FF​ ​(All)
SIP-​ ​10.10.1.1
DIP-​ ​10.10.1.100
Switch1​ ​will​ ​receive​ ​it​ ​and​ ​broadcast​ ​it​ ​to​ ​all​ ​the​ ​other​ ​ports.​ ​Switch1​ ​will​ ​also​ ​add​ ​an​ ​entry​ ​in​ ​its​ ​MAC​ ​forwarding​ ​table,
as​ ​it​ ​has​ ​received​ ​the​ ​request​ ​from​ ​that​ ​MAC​ ​the​ ​first​ ​time.​ ​Destination​ ​server​ ​will​ ​unicast​ ​ARP​ ​response​ ​having:
SMAC-​ ​*00:F2
DMAC-*00:F1
SIP-​ ​10.10.1.100
DIP-​ ​10.10.1.1
Switch1​ ​will​ ​again​ ​add​ ​an​ ​entry​ ​in​ ​its​ ​MAC​ ​forwarding​ ​table​ ​and​ ​forward​ ​this​ ​packet​ ​to​ ​Router1​ ​based​ ​on​ ​look​ ​up.
Router1​ ​at​ ​server​ ​side​ ​will​ ​update​ ​its​ ​ARP​ ​table​ ​and​ ​send​ ​https​ ​data​ ​packet​ ​to​ ​server​ ​having:
SMAC-​ ​*00:F1
DMAC-*00:F2
SIP-​ ​11.1.1.75
DIP-​ ​10.10.1.100
Now,​ ​the​ ​reverse​ ​traversal​ ​from​ ​destination​ ​server​ ​to​ ​the​ ​PC​ ​will​ ​follow​ ​the​ ​same​ ​traditional​ ​terminologies​ ​that​ ​were
followed​ ​while​ ​traversing​ ​in​ ​the​ ​forward​ ​direction.​ ​And​ ​definitely​ ​the​ ​reverse​ ​traversal​ ​will​ ​be​ ​smoother​ ​as​ ​lookups​ ​and
routes​ ​will​ ​be​ ​easily​ ​available.