| cable range | CAC | call admission control | call priority | call set-up time | CBR | CD | CDP | CDVT | cell |
| cell payload scrambling | cell relay | Centrex | CER | channelized E1 | channelized T1 | CHAP | checksum | choke packet | CIDR |
| CIP | CIR | circuit switching | Cisco FRAD | CiscoFusion | Cisco IOS software | CiscoView | Class A network | Class B network | Class C network |
| classical IP over ATM | classless routing | CLI | CLP | CLR | CO | collapsed backbone | collision | collision domain | composite metric |
| configuration register | congestion | congestion avoidance | congestion collapsed | connection ID | connectionless | connection-oriented | console-port | control direct VCC | control distribute VCC |
| convergence | core layer | cost | count to infinity | CPCS | CPE | crankback | CRC | CSMA/CD | CSU |
| CTD | cut-through frame switching | data direct VCC |
data frame | datagram | Data Link control layer | Data Link layer | DCC | DCE | D channel |
| DDP | DDR | DE | default route | delay | demarc | demodulation | demultiplexing | designated bridge | designated port |
| designated router | destination address | directed broadcast | discovery mode | distance-vector routing algorithm | distribution layer | DLCI | DNS | DSAP | DSR |
| DSU | DTE | DTR | DUAL | DVMRP | DXI | dynamic entries | dynamic routing | dynamic VLAN |