Port numbers are divided into three ranges, each managed differently:

Well-known ports are permanently reserved for specific standard services. Here are some important examples:

Note that the same well-known port number is usually registered under both TCP and UDP even if a service uses only one protocol. This avoids confusion across protocols.

On Linux, ports in the range 0 to 1023 are privileged. Only a process with the CAP_NET_BIND_SERVICE capability (traditionally root) can bind to these ports. This is a security measure:

On modern Linux, you can grant just this specific capability to a binary without making it fully root: setcap cap_net_bind_service=+ep /path/to/program

When a client creates a socket and does not call bind(), the OS automatically assigns a temporary (ephemeral) port number. The client does not care which port it uses — it just needs a port for the server to send replies back.

The server knows where to reply because the client’s ephemeral port is included in every packet as the source port. On Linux, the range of ephemeral ports is controlled by:

# View current ephemeral port range on Linux
cat /proc/sys/net/ipv4/ip_local_port_range
# Typical output: 32768   60999

# Change the range (temporary, resets on reboot)
echo "1024 65535" > /proc/sys/net/ipv4/ip_local_port_range

Binding to port 0 explicitly also triggers ephemeral port assignment: addr.sin_port = htons(0); — the OS picks a free port automatically.

#include <stdio.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main() {
    struct servent *svc;

    /*
     * getservbyname() looks up a service by name in /etc/services
     * Returns struct servent with port in network byte order
     */
    svc = getservbyname("ssh", "tcp");
    if (svc != NULL) {
        printf("Service: %-10s  Port: %d  Protocol: %s\n",
               svc->s_name,
               ntohs(svc->s_port),   /* convert to host byte order */
               svc->s_proto);
    }

    svc = getservbyname("http", "tcp");
    if (svc != NULL) {
        printf("Service: %-10s  Port: %d  Protocol: %s\n",
               svc->s_name,
               ntohs(svc->s_port),
               svc->s_proto);
    }

    /*
     * getservbyport() looks up a service by port number
     * Port must be in NETWORK byte order
     */
    svc = getservbyport(htons(443), "tcp");
    if (svc != NULL) {
        printf("Port 443 = Service: %s\n", svc->s_name);
    }

    return 0;
}
/* Output:
   Service: ssh        Port: 22   Protocol: tcp
   Service: http       Port: 80   Protocol: tcp
   Port 443 = Service: https
*/

Finding Your Ephemeral Port After bind(port=0)

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

int main() {
    int fd;
    struct sockaddr_in addr;
    socklen_t addrlen = sizeof(addr);

    fd = socket(AF_INET, SOCK_STREAM, 0);

    memset(&addr, 0, sizeof(addr));
    addr.sin_family      = AF_INET;
    addr.sin_port        = htons(0);          /* 0 = let OS pick */
    addr.sin_addr.s_addr = htonl(INADDR_ANY);

    bind(fd, (struct sockaddr *)&addr, sizeof(addr));

    /*
     * Use getsockname() after bind() to discover which
     * ephemeral port the OS assigned.
     */
    getsockname(fd, (struct sockaddr *)&addr, &addrlen);
    printf("OS assigned ephemeral port: %d\n", ntohs(addr.sin_port));

    close(fd);
    return 0;
}