Startup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.IdentityModel.Tokens;

namespace WebApiJwtExample
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
            services.AddAuthentication(options =>
            {
                options.DefaultAuthenticateScheme = "Jwt";  
                options.DefaultChallengeScheme = "Jwt";              
            }).AddJwtBearer("Jwt", options =>
            {
                options.TokenValidationParameters = new TokenValidationParameters
                {
                    ValidateAudience = false,
                    //ValidAudience = "the audience you want to validate",
                    ValidateIssuer = false,
                    //ValidIssuer = "the isser you want to validate",
                    
                    ValidateIssuerSigningKey = true,
                    IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("the secret that needs to be at least 16 characeters long for HmacSha256")), 
                    
                    ValidateLifetime = true, //validate the expiration and not before values in the token

                    ClockSkew = TimeSpan.FromMinutes(5) //5 minute tolerance for the expiration date
                };
            });
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseAuthentication();

            app.UseMvcWithDefaultRoute();

            app.Run(async (context) =>
            {
                context.Response.StatusCode = 404;
                await context.Response.WriteAsync("Page not found");
            });
        }
    }
}