Cisco router IOS "modes": commands only usable in a particular mode.
exit to backup one mode.
Ctrl-z to jump back to enable mode.

"User mode" when login:  Limited viewing of configuration with 'show'
Router>

Router> enable           (or 'en')  to enter enable mode.
"Privileged/enable mode": (superset of user mode). Can view all configuration 
with 'show' commands.
Router#

************************************
IOS global configuration commands:
"Global config mode": configure system-wide settings, e.g. hostname,
  passwords, default router, static routes...

#  configure terminal        conf t (go into global config mode from exec mode)

Router(config)#

   hostname routersNewName    (change name)

***********************************
IOS interface configuration commands:
"Interface configuration" mode: configure an interface/adapter/port.

From global configuration mode:
   interface NAME      (go into interface config mode)
                     NAME: fa0/0  serial0  s0  s0/0  gi1  etc

Router(config-if)#

   ip address IPADDRESS MASK      (assign IP address and netmask)

   no shutdown                    (bring interface up)

Serial interface being used as DCE end of link of back-to-back routers
in a (simulated) lab needs to have clock rate specified.  
   clock rate BPS                 (BPS: 56000, 1000000 etc)

***********************************
IOS commands to do dynamic routing.

From global config mode:

   router rip           (go into (RIP) routing config mode)

Router(config-router)#

    network IPNETWORK   (a network this routing process is responsible for,
                         i.e. will advertise to other routers its route
                         to this network.
                         RIP is classful, so netmask is implied by class.

***********************************
Show commands:
show ip interface brief    (#1 basic info, but the esim doesn't have!)

show running-config         
show ip route               (the routing table: connected and dynamic routes)
show interfaces [NAME]

*******************************************
#router on a stick: subinterfaces & ISL trunking encapsulation p. 178-
int INT.SUBINT              (802.1Q "native" VLAN is vlan1: no subint)        
(config-subint)#  encapsulation dot1q VLAN#     
(config-subint)#  ip addr ADDR MASK  
Do a "no shutdown" on the interface INT
Subint and Vlan# do not have to be same.