Routers

OSI Layer 3
Connects IP networks together into an internetwork.
Based on IP address: hierarchical (network and host).
Each interface is in different IP network and has valid host IP address in that net.
Forwards packet to next hop in its journey in the internet.
Routing table indicates which interface a packet should be forwarded out.
Routing table created and maintained by a routing protocol.
Routers in the internet communicate among themseleves to converge upon the optimal routes to networks.
Security role: packet filtering firewall block/allow specified types or addresses of traffic.
Traffic management role: packets with unknown addresses dropped.
Separates broadcast domains (broadcasts are not forwarded to other networks).
Edge network has a default gateway: all packets destined to other networks go to it.
RULEs: an (IP) network should have only IP addresses of that network for the hosts of that network (i.e. no interlopers from other IP networks) AND all IP host addresses of that IP network should be in this network (i.e. no host addresses of this IP network should be in other networks).

2600 routers

Wiring closet rack

tracert'ing www.lapl.org:
ae-0.lumen.osakjp02.jp.bb.gin.ntt.net [129.250.8.34]
ae-3-3604.ear1.SanJose1.Level3.net [4.69.214.150]
dc-lax-agg10--lax-agg8-300g.cenic.net [137.164.11.70]

tracert'ing www.bpl.org:
xe-0-0-42-2.a00.sttlwa01.us.ce.gin.ntt.net [168.143.229.150]
e0-0-0-28.pe04.700754thStSE01.YYC.beanfield.com [72.15.49.41]
et49-1.agg01.905KingStW01.YYZ.beanfield.com [72.15.49.86]
199-167-152-168.beanfield.net [199.167.152.168]

tracert'ing www.louvre.fr:
ae-1-3110.edge6.Paris1.Level3.net [4.69.143.174]

tracert'ing www.cenpac.net.nr:
Bundle-Ether-42.br03.tok02.pccwbtn.net [63.218.250.37]
superloop.te0-0-0-13.br03.tok02.pccwbtn.net [63.216.242.214]
bundle-ether34-172.bdr02-ipt-4edenpar-syd.au.superloop.net.co

tracert'ing anadyr.ru  
ae-4.r24.snjsca04.us.bb.gin.ntt.net [129.250.2.118]
ae-2.r24.asbnva02.us.bb.gin.ntt.net [129.250.6.238]
ae-13.r21.parsfr04.fr.bb.gin.ntt.net [129.250.6.5]
ae-11.r21.frnkge13.de.bb.gin.ntt.net [129.250.5.26]